Nitrogen fixation in various Wisconsin lakes was compared using the C2H2 reduction technique. Rates of C2H2 reduction were unaffected by N2 when sufficient C2H2 was added to the gas phase. Thus in field studies the natural gas phase need not be removed before the addition of C2H2. C2H2 was reduced rapidly in eutrophic waters (Little Arbor Vitae Lake, Lake Mendota and southern Green Bay) but more slowly in oligotrophic waters (Crystal Lake, Trout Lake, Lake Mary and northern Green Bay). C2H2 reduction was greatest in the surface waters, was largely light-dependent and showed a marked diurnal variation. There was also significant variation with area and with time, and in general acetylene reduction was detected only when heterocystous algae were present. The rate of nitrogen fixation in Lake Mendota, calculated on the basis of weekly measurements taken during most of the ice-free season, was approximately 2.4 kg/ha/year.