Seasonal forest ponds (SFPs) are isolated, ephemeral lentic habitats in upland forest ecosystems. These ponds occur commonly throughout temperate forests. Faunal communities of these ponds are dominated by invertebrates. Composition of these communities varies temporally both between years and also seasonally within a single hydrologic year, composition is most affected by pond permanence or hydroperiod. Benthic macroinvertebrates (BMIs) were sampled up to three times a year in five SFPs between 1994 and 1996. The ponds were of short, intermediate, and long hydroperiod. Hydroperiod also varied among years, based on precipitation patterns. During the study, 64,000 specimens of 57 taxa were collected. No pattern was identified in the variation of BMI abundance among years and pond hydroperiod; abundance increased with successive surveys within years. Taxon richness and diversity varied significantly with pond hydroperiod, increasing with increasing hydroperiod. Diversity measures increased over the three years of the study but without obvious pattern across the successive surveys within years. Insects dominated the samples, but large numbers of other Arthropoda and Oligochaeta were also collected. Chironomidae were dominant in most ponds, years, and surveys; chironomid dominance was significantly greater in shorter hydroperiod ponds. Seasonal forest ponds function as aquatic islands in a “sea” of terrestrial forest. The effect of hydroperiod on the composition of the benthic macroinvertebrate community is analogous to that of size on marine island fauna, longer hydroperiod ponds generally have richer invertebrate communities just as larger marine islands typically have richer faunas. However, the effect is confounded by the close relationship between pond hydroperiod and pond size/volume.