Gas chromatographic analysis of extracts prepared from adult mayflies (Hexagenia bilineata) collected from the Mississippi River between Prescott, WI and McGregor, IA indicated that polychlorinated biphenyls (PCB's) were as widely distributed and as abundant in 1983 as they were in 1980. The ranges of total body burden of PCB (μg g−1wet weight) within this 300-kilometer section of the river were 0.17—2.10 in 1980 and 0.21—3.10 in 1983. The 1980 geographic distribution pattern of PCB concentrations was similar to that which others found in 1976 forH. bilineata, but the mayfly body burden was less at nearly all sites similarly sampled in 1980. This trend did not persist in 1983; PCB's were as abundant and as widely dispersed inH. bilineatain 1983 as they were in 1976.