Abstract 1. 1. Guppies Poecilia reticutata acclimated to 100% seawater (SW) had lower taurine and alanine levels in muscle than fish kept in freshwater (FW). 2. 2. The glycine level, in contrast, was higher in SW fish than in FW fish. 3. 3. Levels of other free amino acids (FAA) were comparatively low and little different between fish adapted in FW and in SW. 4. 4. In both FW and SW fish almost all of muscle FAA showed little difference in levels between fish kept on diets containing three different levels of trimethylamine (TMA) (0, 223, and 334 μmol TMA/kg dry weight of diet). 5. 5. Total FAA and nonprotein nitrogen levels in muscle were unaffected by the difference in either the diet species or the ambient salinities. 6. 6. Muscle trimethylamine oxide levels were higher in SW fish than in FW fish. 7. 7. In both salinities, muscle trimethylamine oxide levels in fish on the diets containing 223 and 334 μmol TMA/kg were slightly greater than the level in fish on the TMA-free diet.