Cryptosporidium spp. are obligate intracellular apicomplexan parasites that infect epithelial cells of the gastrointestinal systems of a wide range of vertebrate hosts, including humans. Its importance as a serious public health threat was recognized only since the HIV pandemic. Because of the particular filter feeding behavior of bivalves, these marine organisms are susceptible to the accumulation of Cryptosporidium spp. oocysts from the environment and their retention for a certain time, acting as potential zoonotic reservoirs. To preliminary evaluate the presence of Cryptosporidium spp. oocysts in cultured bivalves from the Mali Ston Bay, Adriatic Sea, we have analyzed individuals from a black mussel (Mytilus galloprovincialis) population by immunofluorescence, over a one year period at four different locations. Overall one-year prevalence of the Cryptosporidium spp. was 16.8%, and was correlated with the presumptive number of E. coli in the shellfish and seawater and abiotic factors (temperature, salinity, oxygen), suggesting the necessity for the updating of existing sanitary control measures in Croatia.