The kinds and numbers of arthropods associated with an alfalfa community in New York State were studied. For the whole alfalfa community (591 species), 72 percent of species represented a single species per genus. Single-species-per-genus percentages were not consistently higher for primary members of the alfalfa community comparcd with incidental members. Interspecific competition should have been more intense between primary than incidental members. The calculated species equatability index H' was observed to vary 3- to 4-fold during a season. A new equatability index (E c ) which has many advantages, was employed. The variability of the indices indicates that evenness is not buffered and that it varies in time and probably in space. The conclusion, however, was that neither the species-per-genus nor the species-equatability indices were adequate to describe the species and abundance relationships within the alfalfa community.