The present invention relates to new endoperoxide compounds and compositions, and to a process for producing them for prophylaxis and control of perkinsiosis in bivalves. Endoperoxide compounds with biological activity against Perkinsus olseni include 13 trioxolanes and 9 tetraoxanes. Protozoan parasites of the genus Perkinsus are known to infect several species of marine molluscs worldwide, like oysters, abalones, clams, scallops, pearl oysters, cockles or mussels. The present invention also describes the synthesis of these compounds, in particular of new endoperoxide compounds of the tetraoxane family. Compositions comprising endoperoxide compounds are useful for prophylaxis and control of perkinsiosis in bivalves. Therefore, the present invention also relates to a method of controlling perkinsiosis in bivalves. The present invention is in the domain of aquaculture, medicine, pharmaceuticals and biochemistry.