Acetylene reduction assays in the water columns of several artificially eutrophied lakes in the Experimental Lakes Area revealed that nitrogen fixation by planktonic blue-green algae could supply a significant portion of the nitrogen income to these lakes. In situ 15N2 uptake experiments in one lake indicated that nitrogen fixation was proceeding at a slow rate in the low oxygen region of the thermocline, probably via methane oxidizing bacteria. Other 15N2 uptake experiments in the littoral sediments of an oligotrophic lake failed to detect nitrogen fixation. Generally, algal nitrogen fixation occurred in Shield lakes that were subject to total N:total P loading ratios (wt/wt) of less than approximately 10. This suggests that in these lakes phosphorus removal from loading is the best technique for eutrophication abatement and that nitrogen removal, when applied by itself, may be detrimental because it could encourage bloom formation of nitrogen fixing blue-green algae.Key words: nitrogen fixation, eutrophication, blue-green algae, acetylene reduction, nitrogen-15