Kudoa thyrsites (phylum Myxozoa) is of concern to the British Columbia (BC) aquaculture industry because of its association with post-mortem myoliquefaction in netpen-reared Atlantic salmon ( Salmo salar L.). This condition, commonly referred to as soft flesh syndrome, significantly decreases the market value of affected fish products. The seasonality of K. thyrsites was investigated following natural exposure of Atlantic salmon in seawater netpens at the Pacific Biological Station, in Nanaimo, BC. Atlantic salmon were exposed for several 8-week periods throughout the year, and it was determined that the infections were contracted in only the summer and fall. The progression of these infections in both netpen and tank populations was followed for up to 20 months. The majority of fish contracted K. thyrsites infections. Sporulation typically occurred within 6 months (approximately 2000 degree-days) after transfer to seawater, and the fish almost completely recovered from the original infections within a year. Of 31 nonsalmonid fish species collected from the vicinity of seawater netpens and from research cruises off Vancouver Island, five species were identified as potential reservoir hosts including rock sole ( Pleuronectes bilineatus ), tube-snout ( Aulorhynchus flavidus ), arrowtooth flounder ( Atheresthes stomias ), and two new host records, lingcod ( Ophiodon elongatus ) and threadfin sculpin ( Icelinus filamentosus ). Attempts to transmit the parasite directly from fish to fish by intubation of fresh myxospores were unsuccessful. However, intraperitoneal injection of blood collected from a coho salmon ( Oncorhynchus kisutch ) infected with K. thyrsites successfully transmitted the infection. Using light microscopy to investigate the sequential development, K. thyrsites infections were detected in the somatic musculature at 13 weeks post-exposure (p.e.). In a subsequent experiment, infections were first detected at 6 weeks p.e. using a PCR test and after 9 weeks p.e. using light microscopy. The earliest developmental stage detected by histology was a small plasmodium containing four nuclei at 9 weeks p.e. No host response was observed histologically that was directly related to muscle fibers which contained intact pseudocysts. However, the response associated with ruptured pseudocysts was characterized by chronic, multifocal inflammation between the muscle fibers.