The effects of N2, nitrate and ammonia as N sources were investigated in P-limited and nutrient-sufficient cultures of Anabaena flos-aquae (Lyngb.) Bréb. The maximum growth rate (μm) was highest at 1.34 d−1 with ammonia, compared to 1.18 with nitrate and 0.95 d−1 with N2. There was no difference in P requirement between N2 and nitrate cultures. Under P-limited conditions, the increase in cell P with growth rate (μ) was identical. With N2 as the N source, cell-N concentrations in P-limited cells increased with μ as did cell P, and the cellular N:P ratio remained the same (14) within the range of μ examined. With nitrate, however, cell N concentrations were high and independent of n, except at a low μ. It appears that this organism fixes atmospheric N2 only at the minimum concentration required to maintain a μ. The acetylene reduction rate increased with μ in both N2- and nitrate-grown cells, but the rate was lower in nitrate. Under P-limitation, there was no difference in net C-fixation rate per cell between N2 and nitrate cultures at a given μ. However, the rate per unit of chlorophyll a (chl a) was higher in N2 than in nitrate cultures, and the rate was independent of μ with N2 but was a linear function of nitrate supplied. The maximum C-fixation rate in nutrient sufficient cells was highest with ammonia, followed by nitrate and N2. The cellular chl a concentration was correlated with the total cell-N concentrations regardless of H and the source of N.