On the basis of intensive collections of Microcrustacea (Cladocera, Copepoda, Anostraca, and Notostraca) from 11 sites in the Canadian arctic archipelago, patterns of species richness, distribution, and community composition were evaluated. Microcrustacean communities of pond habitats in the high arctic of Canada are much less diverse than communities at more mesic, subarctic sites. There is both a marked decline in species richness and a dramatic change in the composition of pond communities with increasing latitude and decreasing summer temperatures. Ponds in the low arctic have 3–4 times as many species as those in the high arctic and are dominated by calanoid copepods, a group largely absent from the high arctic. Sites with similar climatic conditions in Alaska have microcrustacean communities with higher species richness, a phenomenon correlated partially with the presence of glacial refuges throughout the Pleistocene. The failure of certain zooplankton taxa, especially calanoids, to colonize areas in the Canadian arctic remote from refuges in the 7000 yr since deglaciation suggests that their dispersal rates are low. Reestablishment of complete zooplankton communities in glaciated terrain with severe environmental conditions apparently requires long periods, although initial colonization by a few pioneer species may be rapid.