The nitrogen and phosphorus content of two temperate fishes, Rutilus rutilus and Perca fluviatilis, and six tropical fishes, Oreochromis niloticus, Cichla monoculus, Serrassalmus rhombeus, Plagioscion squamosissimus, Prochilodus brevis and Hoplias malabaricus, were investigated to test the hypothesis that variation in body P content and N:P ratio is related to body size. Regressions of %P and N:P ratios against fish size (length and mass) confirmed the hypothesis for P. fluviatilis and P. squamosissimus, suggesting that body size is an important factor driving body P content and N:P ratios in some fishes. Moreover, significant increases in %N and N:P ratio with body size was found for H. malabaricus, a common piscivorous fish of the Neotropics. Interspecific variation in %P and N:P ranged two-fold and significant differences (P < 0·05) were found among the tested species. The mean ±s.d. elemental content across all fishes (n= 170) was 10·35 ± 1·29% for N and 3·05 ± 0·82% for P, while the N:P ratio was 8·00 ± 2·14. Data on fish body nutrient content and ratio will improve parameterization of bioenergetics and mass balance models and help clarify the role of fishes in nutrient cycles in both temperate and tropical freshwaters.