Original determinations of the trimethylamine oxide content of 60 species of fish are recorded, and 21 additional species have been studied by others. Elasmobranchs have the highest content of oxide, two to five per cent based on dry weight. Among teleost fishes, the amount increases from the lower to the higher orders, freshwater fish containing no oxide. Analyses of several species of marine invertebrates confirm earlier work showing that certain molluscs, echinoderms and other organisms contain trimethylamine oxide, often in quantities similar to those in the higher teleosts.