Response of Aphanizomenon ovalisporum to certain environmental parameters was studied to gain a better understanding of the conditions which may have stimulated its autumnal bloom in Lake Kinneret. Optimal temperature for A. ovalisporum growth was 26–30 °C, resulting in growth rates of 0·2–0·3 day−1, similar to those observed in the lake. Maximal rate of CO2 fixation (assimilation numbers of 6–8 μg C μg−1 Chl h−1) was obtained at low irradiances (Ik of 40–100 μmol photons m−2 s−1), 200 μM Pi and low N[ratio ]Pi ratios. Growth was strongly affected by phosphorus availability, reaching a maximum at Pi concentrations above 40 μM. The high demand for phosphorus was indicated by an increase in alkaline phosphatase activity. The relative abundance of Pi in the cells increased by 4-fold in Pi-rich compared with Pi-limited cultures. Uptake of Pi was faster in Pi-depleted compared with Pi-sufficient cells. Maximal photosynthetic rates and K1/2(HCO-3) were 140–220 μmol O2 mg−1 Chl h−1 and 10–24 μM, respectively. At pH 7·0 the K1/2(CO2) was 2·2 and fell to 0·04 μM at pH 9·0. These data indicated that A. ovalisporum is a HCO-3 user, and can explain its high photosynthetic rates during the bloom, under high pH and low dissolved CO2 conditions. Na+ concentrations of about 5 mM were essential for A. ovalisporum growth at high pH approaching values in the lake.