Inter-individual differences in the patterns of black and white on the tips of primary feathers 5 through 10 are reported for Blacklegged Kittiwakes (Rissa tridactyla) from Arctic Canada and Newfoundland. Primaries were classified into five types according to the amount of white at the tip. Primaries 5 or 6 (depending on location) were the most variable between individuals and fell more evenly into the five types, compared with primaries 9 and 10, almost all of which were of one type. The Shannon-Weaver index was used to quantify this variation. The shape, number and position of the black patches at the tip of primaries 5 and 6 also varied between individuals, as did the relative size of apical white spots on primaries 6 through 10. These differences could be observed in the field with a spotting scope or binoculars and were used successfully to distinguish between members of the pair at the nest-site with 100% accuracy. Left-right symmetry in wing tip pattern within a bird was high but not perfect. Similarly, patterns were largely, but not perfectly, consistent across two successive wing moults. In conjunction with observations of courtship feeding or copulation, individual differences in wing tip pattern allow the study of birds of known sex at the nest-site, in situations when their capture and marking is undesirable or not possible. Other gull species may exhibit similar variation in wing tip patterns between individuals.