BIOLOGICAL diversity, population size and body size are interdependent(1-8), but there is little consensus on the nature or causes of these relations. Here we analyse the most thoroughly sampled ecological community to date, a grassland insect community sample containing 89,596 individuals of 1,167 species. Each taxonomic order had a distinct body size at which both species richness and number of individuals were highest, but these peak sizes varied more than 100-fold among five major orders, These results suggest that there may be fewer undiscovered small insect species than previously thought. Moreover, we found a surprisingly strong, simple, but unreported, relation between species richness (S) and the number of individuals (I) within size classes, S =I-0.5. Because this held across numerous body types and a 100,000-fold body-size range, there may be a general rule that is independent of body size for the relations among interspecific resource division, abundance and diversity.