The distribution and ecology of Cladocera and Copepoda in 144 Lake District tarns showed that although some species appear to be ubiquitous, there is a strong association between the distribution of many species and the presence or absence of aquatic weeds. Fifty-three species of Cladocera and twenty-eight species of Copepoda were found, all of them in tarns widely scattered geographically. Only a few species were found in a high proportion of the tarns, and nearly half were distributed among ten or fewer tarns. Notes on the ecology of individual species are given.