Fish are important in the structuring of other communities and may have large effects on the functioning of aquatic ecosystems. The structure of fish communities, in turn, seems to differ with climate. We compared the characteristics of fish assemblages in lowland streams located in two contrasting climates (cold-temperate Europe and subtropical South America) by use of published and unpublished data on streams of similar depth, width, and slope (n total = 91 streams). We also selected a subset of seven comparable little-affected streams in the two contrasting climates: temperate (Denmark, 55°–57°N, Dk) and subtropical (Uruguay, 30°–35°S, Uy) and compared the fish community structures in relation to environmental characteristics. We then analysed a series of potential explanatory factors behind the patterns observed, in particular the effect of ambient temperature, by comparing temperature-corrected community metabolism. Significantly higher species richness, higher densities, lower biomass, smaller mean body size, and lower mean weight of fish were observed for the subtropical streams than for the temperate streams, both in the literature review and in the subset of streams. Several characteristics of fish assemblages in streams may be explained by direct and indirect effects of temperature. Accordingly, fish in subtropical systems had a temperature-corrected community metabolism I m−2 equal to that of fish in temperate systems, indicating that temperature, besides historical factors, is an important driver of different size structures. Our findings concur with differences previously found in littoral areas of shallow lakes, suggesting that these patterns are not restricted to running waters. Our results elucidate how fish community structure might be affected by increases in temperature triggered by climate warming.