Juvenile mortality was measured, and mortality factors were studied, in an island colony of 850 pairs of Ring-billed Gulls in northern Michigan. Thirty-seven carcasses of dead juvenile gulls were collected in plots covering 36 square meters of the colony area, indicating a total mortality, when corrected for subsequent deaths, of about 1.50 chicks per nesting territory. All size classes were represented among the carcasses collected, but the highest mortality apparently occurred in middle-sized birds, those large enought o move away from the nest but too small to defend themselves effectively against adult attack. Counts of chicks on two study plots revealed about 0.67 survivors per territory at fledging. Comparison with published data on the Herring Gull suggests that this low production rate may be adequate to maintain the population and hence normal for the species. Vigorous defense of the small, tightly-massed territories against both young and old intruders was the direct cause of the great majority of the juvenile deaths. Predation by Herring Gulls was a secondary factor. Wandering of young from their territories during the absence of the parents indirectly contributed to juvenile mortality by necessitating returns for feeding through the defended territories of other birds. Destruction of shading vegetation and intrusion of the colony by human observers induced forced movements of chicks and thus aggravted the conditions producing mortality.