ABSTRACT. Ever since its first description the paramyxean parasite Marteilia refringens has been recognised as one of the most significant pathogens of bivalve molluscs. The existence of a complex life cycle has been postulated by many authors. However, the transmission route of A4, refringens and its life cycle remain unresolved. Experimental observations are presented here that may help lead to a satisfactory hypothesis of the M. refringens life cycle. Trials were performed to test the ~nfectivity of purified early and late stages of M. refringens by different routes of inoculation. Horizontal transmission was also tested by cohabitation with carriers in experimental tanks. Field cohabitation and exposure to natural sources of the parasite helped examine the role of the environment in the transmission of M. refringens. Lastly, naturally infected oysters were monitored in experimental tanks, in order to study infection progression in the oysters through serial samples. These experiments provide no evidence to support direct horizontal transmission of M. refringens, but rather suggest that intermediate or alternative hosts, or free-living stages, are essential in the life cycle of the parasite.