The in situ acetylene‐reducing (nitrogen‐fixing) activities within the sediments of 2 eutrophic lakes were examined to determine whether this process plays a significant role in enhancing nutrient enrichment of these lakes. Estimates of the N‐fixing potential on a sediment dry‐weight basis were 10.9–112.1 ng g‐1h‐1or on an area basis were 45.3–99 ng m‐2h‐1. The distribution of C2H2reducing activity within the sediment paralleled the distribution of Kjeldahl N, total organic carbon, and microbial biomass and was inversely related to NH+4—N in interstitial water. A seasonal variation in the activity was observed in the top most sediment strata, with the highest rates occurring during winter. Estimates of the total N input reveal that this is of little quantitative importance compared to inputs of combined N from other sources.