The actinosporean life stage of Ceratomyxa shasta, a myxozoan parasite of salmonids, and the annelid worm that serves as its alternate host were identified in laboratory transmission experiments and their roles were confirmed using molecular techniques. Infection by the parasite occurred in susceptible fish that were either exposed to or force fed the freshwater polychaete, Manayunkia speciosa, infected with the actinosporean. These observations were confirmed using the polymerase chain reaction with primers designed from the C. shasta 18S rDNA sequence. DNA was amplified from polychaetes harboring the actinosporean that caused infection in the fish but not from uninfected polychaetes. Amplified DNA from an infected polychaete was sequenced and its homology with the 18S rDNA sequence of C. shasta spores verified the proposed life cycle. Ultrastructural examination of the actinosporean in the polychaete showed developmental stages in the epidermis rather than within the intestinal epithelium as described for other myxozoans. The methods described will be useful in identifying alternate hosts and morphologically diverse life stages in the complex life cycles of other myxosporea and in understanding the relationships between these parasites and their hosts.