The autecology of the estuarine alga, Blidingia minima var. subsalsa (Kjellman) Scagel (Chlorophyceae) was considered with regard to its growth, reproduction and distribution. Studies were conducted on the Squamish River estuary, British Columbia, from May 1974 to July 1975 and in the laboratory from January to August 1975. The major environmental factors considered were light, temperature, salinity, nutrients and desiccation. The alga occurred on the Squamish River delta in the upper intertidal zone. Biomass, as estimated by percent cover, increased in the period March to early May, then remained stable or decreased during spring runoff in May and June, finally increasing to a maximum in August. Increases in percent cover were associated with brackish salinity, high light intensity, high air temperature, considerable desiccation, an absence of algal competitors and possibly favourable ion ratios. The three factors investigated in the laboratory (temperature, salinity and nutrients) interacted in their effect on net photosynthesis. Reproduction in the laboratory occurred by release of quadri-flagellate and isomorphic biflagellate swarmers. Some biflagellate swarmers fused and germinated, producing isomorphic plants. The period of swarmer release was from January to early May. The alga was perennial but during the winter only its prostrate basal disc was present. Maximum vertical distribution was from 1.5 to 4.0 m above chart datum (lowest low water). Maximum percent cover occurred at ca. 3.25 m. The upper limit appeared to be associated with unfavourable osmotic conditions, e.g. rain or desiccation, and the lower limit with low light intensities. Horizontal distribution was limited by absence of salt water on the freshwater side and competition from Fucus distichus subsp. edentatus (De la Pylaie) Powell on the marine side of the estuary. Blidingia minima var. subsalsa1s geographical and habitat distribution was also examined by reference to literature reports and herbarium collections. The species is cosmopolitan, occurring in every ocean except the Indian and Antarctic with most reports from polar and temperate regions. The variety occurs in brackish, marine and freshwater habitats. / Science, Faculty of / Botany, Department of / Graduate