In situ surveys using scuba are important tools in the management of smallmouth bass (Micropterus dolomieu) in boreal, clear lakes of North America. Population estimates that are derived from such surveys may be erroneous if the size composition of the fish observed by divers differs from that of the entire population. Forty-four adult smallmouth bass of three size classes were radio-tagged and tracked during the summer of 1993 (mid June to early September) in Green Lake, Maine, USA, to investigate possible size-related error from observations by divers being towed along the lake shore. Our results indicate that scuba divers may fail to count a significant portion of large smallmouth bass during late summer (mid July to early September), compared to small- or medium-sized fish. The results suggest that scuba surveys should be conducted during early summer (mid June to mid July) to derive more accurate population estimates.