The effects of low-temperature (0°C) incubation on nitrogenase-catalyzed acetylene reduction, alkaline phosphatase activity and cellular ATP pool(s) have been studied in Nostoc linckia and Nostoc muscorum. The cultures incubated at 0°C showed higher enzymatic activities than those incubated at 25°C but the ATP content was lower at 0°C. Our experiments with chloramphenicol and rifampicin suggest that de novo protein synthesis is required for nitrogenase, but not for alkaline phosphatase even at low temperature. The increase in alkaline phosphatase activity at 0°C may possibly be due to enhanced catalytic activity of this enzyme. The ability to protect enzyme activities at 0°C implies that N. linckia and Nostoc muscorum may be tolerant to low-temperature cellular damage.