A two-year study of the actinosporean fauna of oligochaetes was conducted at an Atlantic salmon fish farm located at the extreme north of Scotland. The actinosporean fauna and their morphological characteristics, the ultrastructural development of four different actinosporean collective groups, the epidemiology of all actinosporean types identified,the complete life cycle of Sphaerospora truttae, the circadian and seasonal spore release patterns of actinosporean types and the myxospores of S. truttae, the viability of actinosporeans and their responses to fish mucus were determined. Twenty one actinosporean types belonging to seven collective groups: Synactinomyxon (3 types), Aurantiactinomyxon (4 types), Echinactinomyxon (5 types), Raabeia (6 types), Neoactinomyxum (l type), Triactinomyxon (1 type) and Siedleckiella (1 type) are described. Six types were identified to previously described forms; Synactinomyxon "A" of McGeorge et al. (1997); Synactinomyxon tubificis Stole, 1899, S. longicauda Marques, 1984, Aurantiactinomyxon-type of McGeorge et al. (1997), Echinactinomyxon radiatum Janiszewska, 1957, Raabeia-type of McGeorge et ai. (1997). The remainder appeared to be new types of the collective groups. Temperature was found to have a significant effect on the spore morphology and caused statistically important differences in the spore dimensions, especially on the caudal processes. Synactinomyxon-type 1, Aurantiactinomyxon-type3, Echinactinomyxon-type5 and Raabeia-type4 were studied at the TEM level to determine the developmental stages of each type. All actinosporean types studied had uninucleate cells as the earliest stage of development. Formation of a subsequent binucleate cell stage was either due to the division of the nucleus in a uninucleate cell or the plasmogamy of two uninucleate cells. The earliest pansporocyst formation seen was two outer somatic cells surrounding two inner generative alpha and beta cells in all actinosporean types studied. However, the formation of an early pansporocyst followed a four-nuclei stage only in Raabeia. Subsequently, the number of somatic and generative cells increased as a result of mitotic divisions and reached 8 alpha and 8 beta cells at the end of the division stages. Echinactinomyxon had only four somatic cells in pansporocyst, whilst Synactinomyxon, Aurantiactinomyxon and Raabeia had eight. Following the copulation of each pair of alpha and beta cells, 8 zygotes were formed. Then, two mitotic divisions of each zygote resulted in a four-cell stage of each sporoblast. Valvogenesis and capsulogenesis was followed by the formation of 8 mature spores inside each pansporocyst. Over the two year sampling programme the overall infection prevalence of oligochaetes with actinosporeans was 2.9%. The infection prevalence was higher in the first year (3.3%) than the second year (2.3%). The infection prevalences of individual types were between 0.001% and 0.9%. Summer was the preferred season of spore release (4.1%), followed by autumn (2.9%) , spring (2.8%) and winter (1.6%), Some parasites such as Echinactinomyxon-typel released spores throughout the study period, whilst Synactinomyxon-type2 was recorded only in summer. There was also a positive relationship between the number of actinosporean types released and water temperature. A one year sampling programme also indicated that Sphaerospora truttae had two distinct life cycle phases, extrasporogonic and sporogonic, in the fish. Extrasporogonic stages were first detected at the beginning of July 1996 with a prevalence of 50% and were seen over an 8-10 week period. Sporogonic stages first became detectable in the kidney tubules at the beginning of September 1996. As well as sporogonic stages, many developing pseudoplasmodia were also observed at this time. Pseudoplasmodia were always present along with mature spores. The infection prevalence stayed above 80% throughout the period of infection. Experimental infections showed that Echinactinomyxon-type5, was the alternate life cycle stage of S. truttae in the oligochaete Lumbriculus variegatus. The time taken from the exposure of Atlantic salmon to Echinactinomyxon-type5 spores to formation of mature Sphaerospora truttae spores was 4.5 months (138 days). However, infections of Atlantic salmon with presporogonic and immature spores of S. truttae were first seen at 3.5 months post-exposure (110 days). In addition to S. truttae, the life cycle of Chloromyxum truttae was also completed at 4.5 months (138 days) post - exposure at 12-16°C using Aurantiactinomyxon-type4 spores released from Tubifex tubifex. Worms infected with Synactinomyxon-type 1, Aurantiactinomyxon-type I, Echinactinomyxon-type1 and type5, Raabeia-type4 and Neoactinomyxum-type showed inconsistent spore release patterns over five subsequent days at ambient temperatures. Up to 5000 spores an each day were released from infected worms with the exception of Echinactinomyxon-type5 which released up to 80,000 spores per day. Experimentally there was a positive relationship between the numbers of spores shed and water temperature. The spore release of worms infected with Synactinomyxon-type I, Aurantiactinomyxon-type 1, Echinactinomyxon-type I, Raabeia-type4 and Neoactinomyxum-type spores were also studied at 3 h intervals and showed that peak release occurred between 22.00 and 01.00 h. Studies on the spore release patterns of Sphaerospora truttae myxospores from Atlantic salmon showed that mature spores were first released at the end of November, peaked around April and then decreased sharply. Number of mature spores present in the kidney of the fish showed a similar pattern of abundance. Polar filaments of Echinactinomyxon-type I, Raabeia-type4 and Aurantiactinomyxon-type I spores discharged in response to mucus from Atlantic salmon, brown trout, 3-spined stickleback and common carp. However, the response to the mucus from each fish species was different. In each case majority of discharges occurred within the first 5 min of exposure to mucus although there were further discharges up to lh. The viability of Synactinomyxon-type I, Echinactinomyxon-type I, Raabeia-type4, Aurantiactinomyxon-typel and Neoactinomyxum-type spores had a negative correlation with increasing temperature. In general, the spores remained viable for 6-7 days at 4°C, 4-5 days at 13°C and 4 days 22°C.