In the framework of a one-year survey, the actinosporean infection of the oligochaete fauna living in the mud and on the vegetation of Lake Balaton, Hungary was studied. A total of 10 types of actinospores were isolated from the oligochaetes Tubifex tubifex and Limnodrilus hoffmeisteri collected during the survey, which could be classified into the triactinomyxon, raabeia and aurantiactinomyxon groups. The drawings depicting the individual actinospore types are presented on a plate and their characteristic dimensions summarised in tables. The prevalence and seasonality of actinosporean infections observed in the two oligochaete species are illustrated graphically. Infection by actinospores showed pronounced seasonality. In the spring and summer the prevalence of triactinomyxon infection in T. tubifex exceeded 30 and 40%, respectively, while in the autumn it dropped to 6%. In T. tubifex raabeia infection of 3% prevalence was detected only in the summer period. In L. hoffmeisteri the peak of aurantiactinomyxon, raabeia and triactinomyxon infection occurred in the summer with a prevalence of 21, 10 and 8%, respectively. Actinosporean infection in the individual Oligochaeta species showed higher prevalence values than had been reported in the literature, which fact may be explained by the novelty of the examination technique used. Four types of actinospores found in Lake Balaton are identical with the actinospores already described. It cannot be determined with absolute certainty which stages of myxosporeans parasitic in Lake Balaton fishes are identical with the 10 actinospores described.