Acetylene reduction by detached nodules ofAlnus cordata, A. glutinosaandColletia paradoxawas tested with respect to the effect of temperature on nitrogenase activity. Nodules were incubated at temperatures from 5°C to 45°C, exposed to C2H2generally for 1 hr, and sampled for C2H4. The maximum of activity occurred in all three species tested at about 25°C. The C2H2reduction rapidly declined at temperatures above the optimum and stopped at 40°C forA. glutinosaand 45°C forA. cordataandColletia paradoxa. Nodules of.A. cordatasubjected to 5°C recovered nitrogenase activity within 1 hr when the temperature was raised to 25°C, whereas loss of activity was irreversible at 45°C.A. cordatanodules tested in different seasons of the year showed similar activities in April and October with a slightly lower rate in August and a very much lower activity in January.