Fish communities in circumneutral Wisconsin lakes contained significantly more species than did those in acidic lakes (pH 5.1-6.0). Common, as well as rare, species occurred with lower frequency in acidic lakes than in circumneutral lakes. Certain taxa, such as minnows and darters, were either absent or rare in the acidic lakes, probably because of pH-related stress. The differences in species composition and richness of fish communities between acidic and circumneutral lakes did not appear to be related to differences in physical habitat characteristics, past fish migrations or productivity between the two lake groups.