I measured water quality variables, primary productivity, biomass of phytoplankton palatable to zooplankton and zooplankton abundance in mesotrophic Hugo Reservoir, Oklahoma (U.S.A.) at weekly to biweekly intervals between March and September, 1996. Canonical Correspondence Analysis (CCA) was used to estimate the influence of abiotic factors and resource availability in structuring the zooplankton assemblage over the season. Repeated Measures Analysis of Variance (ANOVA) was used to determine spatial and temporal variation in zooplankton abundances and to make comparisons between repeated measures ANOVA and CCA in analyzing the zooplankton assemblage. In addition, I analyzed relationships among cladoceran reproductive parameters (egg ratio, instantaneous birth rate, reproductive ratio and mean brood size) and environmental factors over the season using product moment correlation. Results showed little evidence of bottom-up control of the zooplankton assemblage. CCA and correlations suggested temperature was the most important factor regulating assemblage structure over the season. Several inverse relationships were seen among cladoceran reproductive parameters and temperature. Declines in Holopedium gibberum Zaddach, Bosmina longirostris Muller and Ceriodaphnia lacustris Birge during June corresponded with increases in mean temperature to above 25 °C. Increases in birth rates at the time of population declines suggested that population declines were a result of increased mortality rates (likely due to high water temperature or predation). Spatial variation in the zooplankton assemblage coincided with differences in both temperature and turbidity among sites. Lower abundances of Daphnia parvula Fordyce and C. lacustris and a higher abundance of rotifers at the up-reservoir site coincided with higher mean temperature of the water column and higher turbidity than down-reservoir sites. CCA and ANOVA showed similar results for the spatial and temporal variation in the zooplankton assemblage while CCA provided a more clear approach for analyzing the effects of environmental variables on the assemblage.