Aim To document continental- and regional-scale variation in the size distributionsof freshwater fish and examine some energetic, evolutionary and biogeographicexplanations for these patterns.Location North America.Methods Regional species lists, coupled with habitat and body size information,were used to document the spatial patterns.Results At the continental scale, riverine specialist fishes show a unimodal, rightskewed,body size distribution whereas habitat generalist and lacustrine specialistspecies exhibit bimodal size distributions, with only a slight preponderance ofsmall-mode species. Most large-mode species are migratory. Resident species,unlike migratory ones, show a latitudinal increase in mean size, but the size increaseacross all species is steeper because the importance of large migratory speciesincreases with latitude. Size distributions change from right- to left-skewed withincreasing latitude. Maximum body size does not change with increasing familyrichness but minimum size declines and skewness increases, consistent with diversificationof small species. Skewness does not vary with mean family body size.Main conclusions Post-glacial recolonization by large, habitat generalist, migratoryspecies is the main determinant of latitudinal size distribution trends. There islittle support for the energetic hypothesis, but the data are consistent with a negativeCope’s rule.