A contrast between temperate and tropical Cladocera, Copepoda and Rotifera zooplankton fauna shows a markedly different size composition and species diversity. There are fewer species, and these are generally smaller in the tropics than in either the North or South Temperate Zones. Larger species of the genera Daphnia and Simocephalus are relatively rare in the tropics, while members of Eurycerus and Saycia are absent. Members of the families Holopedidae, Leptodoridae and Polyphemidae increase in abundance toward the temperate regions. Two small cyclopoid copepods are common in tropical zooplankton, while many of the common larger species of other regions are absent or very rare. The chief rotifer components are Keratella tropica and species of Brachionus.