Recent studies have hypothesized that differences in elemental composition of zooplankton (N: P ratio) are related to differences in specific growth rate, as increased growth rate requires greater ribosomal RNA, which should result in increased %P and decreased N : P. This hypothesis was tested interspecifically using zooplankton taxa with different specific growth rates and intraspecifically using different ontogenetic stages within single species. Among all five species, growth rate was positively correlated with %N and %P and negatively correlated with N : P. However, %N increased only from ~8% to ~9% while %P varied fourfold across the observed range of growth rates, indicating that changes in %P drive the negative relationship between growth rate and N : P. Intraspecific comparisons showed that growth rate was positively related to %P and negatively related to N:P for Daphnia lumholtzi, but growth rate was unrelated to %P and N : P for D. magna and D. obtusa. Mean %P, %N, and N : P differed significantly among Daphnia spp. and among populations of Scapholebris mucronata and Bosmina longirostris. These data strongly support the growth rate hypothesis as an explanation for differences in body N : P in zooplankton and indicate that the biological stoichiometry of a rapid growth rate life-history is phosphorus intensive.