1. The occurrence and distribution of twelve dehydrogenases have been studied histochemically in two species of rhabdocoel turbellarians entosymbiotic in marine bivalves. 2. Both species (Paravortex scrobiculariae in Scrobicularia plana and P. cardii in Cerastoderma edule) possess dehydrogenases concerned with the three major energy-producing pathways. 3. P. scrobiculariae shows a much greater emphasis on glycolysis than does P. cardii. 4. It is suggested that the emphasis on glycolysis in P. scrobiculariae is an adaptation to regular variations in oxygen supply in its habitat; this adaptation facilitates feeding and associated migrations within the host which occur at times when the host is not submerged by the tide and when oxygen is likely to be in short supply. 5. Dehydrogenases associated with the pentose phosphate shunt are especially evident in the developing embryos of both species and are probably concerned with synthesis of nucleotides and nucleoproteins.