Data on the content, chemical form, and distribution of mercury and selenium in edible muscle are presented for several species of marine and freshwater fish. For most species, 60 to 95% of the total mercury content is present as methylmercury. For all species, 15 to 35% of the total selenium content is in the form of selenate ( Se VI). Muscle selenium content does not correlate with the corresponding mercury content. For freshwater and processed (canned) marine samples, 60 to 90% of the total mercury content is water-extractable. On a percentage basis, methylmercury is slightly more extractable than inorganic mercury. For nonprocessed marine samples, only 25 to 45% of the total mercury is water-extractable, inorganic mercury being more extractable than methylmercury. For all species, 55 to 80% of the total selenium content is water extractable, Se VI is more extractable on a percentage basis than selenite (Se IV) and selenide (Se-II).