Current research has demonstrated that a guild of at least a dozen diatom genera dominates the autotrophic base of the systems in the Perkiomen and Skippack Creeks in Montgomery County, Pennsylvania. Previous research in the Skippack alone demonstrated that the common genera exhibited distinct periodicities and suggested that total diatom densities were similar from sampling dates centered around the autumnal and vernal equinoxes. We investigated the question of whether the diversity, richness, and equitability of the diatom guild were the same or different at the equinoxes. Collections at fourteen day intervals centered around the equinoxes of 1992–3 and 1994–5. Densities per date were determined by Whipple field counts, and analyses of diversity, richness, equitability, and community similarity were made using Simpson's Index of Diversity and Sorensen's Coefficient of Similarity. We found that the diatom communities are not the same at the equinoxes, including their total densities and dominant genera. Generic richness varied little between seasons, but the systems were more diverse at the vernal equinox because of changes in equitability.