One major tenet of Ecological Stoichiometry (ES) theory states that the evolution of life history traits can be influenced by constraints related to nitrogen or phosphorus requirements of these traits.  Therefore, ES can provide a link between the evolution of life history traits and biogeochemical cycling of N and P.  For instance, the growth rate hypothesis (GRH) predicts that variations in C:N:P stoichiometry between taxa can be explained by differential allocation to the machinery of cellular protein synthesis, specifically allocation to P-rich rRNA.  Our objective was to see if the GRH could explain intraspecific variation in growth rate in the ciliate T. thermophila by measuring cell mass, %C, %N, %P, DNA P, and doubling time of cell line populations as a proxy for growth rate in 5 wild-type and 3 domesticated lab cell lines.  Our results were inconsistent with the GRH in that %P, N:P, and C:P did not  explain variation in doubling time.  However, the mass of P/cell was significantly related to doubling time.  Thus, we found that for T. thermophila, growth rate is both a function of cell size and P content.  Variation between wild-type lines suggests there are potentially ecologically important intraspecific differences in the impact these organisms may have on P biogeochemistry.