Vitamin B12 concentrations, measured as combined, uncombined, and total, were determined in Pandapas Pond, Virginia, June through December 1977. Average summer vitamin B12 concentrations (all forms) were higher than during the fall and winter. Mean concentrations of combined, uncombined, and total vitamin B12 were 2.3 ng/l, 3.5 ng/l, and 5.4 ng/l, respectively. Combined vitamin Bl2 usually comprised more than 50% of the total vitamin B12 concentrations. Vertical distribution of vitamin 12 appeared variable, however, positive correlations (p≤0.05) were found between combined vitamin B12, total vitamin B12, and depth. Mean chlorophyll A concentrations were 0.0007 mg/m³ in Pandapas Pond, June through December 1977. The correlations and seasonal trends, high vitamin Bl2 concentrations and low chlorophyll A concentrations, suggested that the phytoplankters (as indicated by chlorophyll A) are primarily utilizers, not producers of vitamin Bl2. Zooplankton were quantitatively and qualitatively analyzed during the same period. Rotifers were the most abundant zooplankton, Keratella cochlearis and Polyarthra vulgaris were the predominant rotifers. Seasonal densities of K. cochlearis and P. vulgaris followed uncombined vitamin Bl2 concentrations directly for the former and with a one to two week lag for the later. Positive correlations (p≤0.05) were found between uncombined vitamin Bl2 concentrations P. vulgaris, Trichocerca sp., T. capucina, and T. multicrinis. The population dynamics of the rotifers in Pandapas Pond were, in part, influenced by concentrations of vitamin B12. However, correlation and stepwise regression analyses indicated that other factors, such as pH, temperature, and cations, also affected rotifer populations in the pond. / Master of Science