The spotted minnow, Galaxias maculatus has a widespread southern hemisphere and circum-polar distribution including south-western and south-eastern Australia. It was sampled at monthly intervals over 12-18 months, by seine and plankton netting at three localities including a freshwater lake, Moates Lake, and two intermittently flowing, naturally saline rivers, the Jerdacuttup and the Oldfield rivers on the south coast of Western Australia. The resulting data provided an opportunity to describe the biology of G. maculatus in some detail including; environmental variables, life cycle, larval development, diet and parasitism by platyhelminth and nematode worms. Comparisons were made with other studies in south-east Australia, including Tasmania, and New Zealand. The present study confirmed that, at least throughout most of its range in Western Australia, G. maculatus has established a self-sustaining land-locked reproductive strategy. It is hypothesised that the development of land-locked breeding is an adaptive response to changing coastal geomorphology in the Holocene period that restricted ocean access of rivers and their fauna and caused estuaries to become non-tidal. The principal conclusion arising is that the local biology differs largely in degree rather than kind from elsewhere it has been studied; differences in degree are interpreted as local adaptations to an environment that is both variable and unpredictable Field measurement of environmental variables revealed G. maculatus will withstand salinities to approximately 46 ppt and surface water temperatures to 280 c. Very low dissolved oxygen concentrations to <1.0 mg r1 are accommodated by practicing secondary aerial respiration at the water surface. Galaxias maculatus on the south coast of Western Australia were smaller than those reported from populations elsewhere. Overall tota11engths of Western Australian males and females ranged 23-132 mm compared to 38-187 mm length to caudal fork for south-west Victoria, 31-185 mm standard length for Tasmania and 40-152 mm length to caudal fork for New Zealand fish. In the present study, size varied between the lake and one river population that was smaller. It is hypothesised that reduction in size of Western Australian G. maculatus generally is an adaptive response to avoid predation by piscivorous birds in shallow, confined river pools and lakes. There was a well defined, albeit extended, breeding season between autumn and spring with peak spawning in winter. The season was longer in the relatively stable lake situation and shorter ~ the very variable river situation partly due to the influence of river flow, which is continuous into the lake and intermittent and variable in the rivers. A flow dependent upstream spawning migration was part of the reproductive strategy but there was also the capacity, in certain circumstances, of spawning on falling water levels in years of nil or little flow. There was an almost complete cessation of reproductive activity during summer. Fecundity ranged from 296-2 874 eggs with a mean of 912 and was positively correlated with total length. The overall total lengths at which 50% of females and males and attain sexual maturity were estimated at 52 and 49 mm total length, respectively. For 95% of females and males the total lengths were estimated at 74 and 62mm total length, respectively. Ageing by counting annual growth rings was successful for lake inhabiting fish only, the lack of consistency in growth rings in the river environments was attributed to the extreme variability of these environments. The von Bertalanffy growth equation predicted that, on average, at the end of their first, second and third years females were 61, 81 and 88 mm total length respectively. Male predictions were 56, 74 and 80 mm, respectively. Approximately 75% of males and 62% of females attained sexual maturity at the end of their first year. Excluding larval fish, 73.1, 22.7, 4.1 and 0.1 % were 0+, 1+, 2+ and 3+ fish, respectively. The overall sex ratio females:males was 1.09:1.0, the ratio favoured males for very small fish but favoured females as fish aged and grew. Larval development was described in detail for the first time for Australian G. maculatus. The sequence of fin development was the same as that reported for galaxiids elsewhere, i.e. caudal, dorsal, anal, pectoral and pelvic. Adult fin ray counts were; cauda1 16, dorsa1 9, anal 13, pectoral 12 and pelvic 6-7. Myomeres ranged from 45-50. Development of pigmentation and dentition were described; caniniform teeth began to develop during the late postflexion larval stage. Dietary analysis confirmed a previous description of G. maculatus as an euryphagic carnivore. A wide range of invertebrate food groups including copepods, amphipods and ostracods, aquatic insects as well as terrestrial invertebrates (spiders, winged ants and orthopterans) were consumed. Most variation in diet was explained by site, i.e whether fish were from river or lake environments or which river environment. A lack of replicate samples precluded a rigorous statistical analysis of the influence of either fish size or season on diet. However, a provisional analysis suggested these variables have minimal influence. Larval diets comprised copepods, cladocerans and unicellular algae; with the attainment of postflexion larval stage and development of caniniform dentition, a wider range of dietary items were ingested. One cestode, one trematode and two nematode larval worms infected river and lake inhabiting fish. The cestode, Ligula intestinalis, infected 13% of lake inhabiting fish causing gross disfiguration and probably reduced reproductive success, particularly of males. The degree and severity of cestode infection was much less in rivers, perhaps due to their saline waters. The worms' adult hosts in all cases were piscivorous waterfowl particularly the white-faced heron. At present G. maculatus is widespread and abundant throughout its range in Western Australia. As most of its range is in rivers and lakes which are, and will in the future be influenced by clearing for agriculture, it is likely that increased river recharge due to clearing will initially benefit G. maculatus. However long term change, particularly changes to riparian vegetation structure and species composition, are likely eventually to be inimical as the shading value of vegetation and its habitat value for terrestrial invertebrate food are diminished.