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Studies of rotifer community composition and dynamics often rely on limited sampling regimes. To determine how well species richness is reflected in these studies, we examined interannual variation of rotifer species richness and monogonont community structure from 10 aquatic systems comprising four habitat types-springs, rock pools (tinajas), former cattle tanks, and artificial ponds-in Big Bend National Park (Texas, USA). Planktonic, littoral, and benthic samples were collected from all sites at about the same date for each of five summers (2001-2005). Our survey yielded 15 monogonont families including 30 genera and 84 species. Two bdelloid taxa also were designated. Species richness varied widely among these four habitats: range, 1-32; mean (+/- 1 SD), 11.2 +/- 8.0. Total Species richness in the habitats also varied considerably: springs (54 taxa) > artificial ponds (35 taxa) > tinajas (19 taxa) > cattle tanks (15 taxa). Sessile species comprised approximate to 13% of the taxa in our samples. Species turnover indices (STI) of these systems indicate low overall relatedness: mean (+/- 1 S.D.) = 85.2 +/- 7.1%. The relative frequency of encounter of most taxa in the four systems was low, with 79 taxa (approximate to 92%) having values <= 2.0%. Singleton rates were quite high, ranging from 46.7 to 71.4%, with an overall mean approximate to 65.1%. Most importantly, we found that both species richness and STI varied considerably among habitat type. Species richness varied by 2-10x between consecutive years and STI ranged from 64 to 89% over the entire study. Our results indicate that rotifer community composition fluctuates greatly over time, and that rotifer community structure may be more labile than is generally believed. Species richness and thus biodiversity may be dramatically underestimated using single sampling or short-term strategies that are often employed in studies of zooplankton community structure.