A field experiment in an experimentally eutrophied lake (Lake 227) tested the hypothesis that differential recycling of nitrogen and phosphorus by Daphnia (a zooplankter with low body N:P ratio) affects the physiological status of cyanobacteria, including rates of N, fixation (stoichiometric recycling hypothesis). After a 5-d incubation in 2.4-liter bottles, Daphnia treatments had lower standing stocks of algae (based on particulate carbon or Chl a) and higher concentrations of dissolved nutrients than did Epischura (a high N:P ratio consumer) or control treatments lacking macrozooplankton, The N:P ratio of dissolved nutrients was higher in the Daphnia treatments than in control or Epischura treatments, consistent with the stoichiometric recycling hypothesis, and was associated with greater algal P deficiency. Measurements of absolute and C-specific N2 fixation rates following the incubation showed that Daphnia treatments experienced a 50% reduction in N2 fixation relative to Epischura and control treatments. This reduction is consistent with the higher N: P ratio of the dissolved pool in the Daphnia treatment, as NH4-N inhibits N2 fixation. Thus, by differentially recycling NH4-N relative to P, Daphnia reduce the advantage (N: fixation) cyanobacteria have over other phytoplankton. We suggest that this mechanism may be an important factor in the ability of Daphnia to reduce the incidence of cyanobacterial blooms in lakes with low N:P loading ratios.