We examined several morphological characters of adult Black-legged Kittiwakes (Rissa tridactyla) breeding in Brittany, France, near the southern limit of the species’ range. Males were significantly larger than females in body mass, head length, wing and tarsus, and the French population differed highly significantly in size from previously studied populations in England and Alaska. There was a strong pattern of assortative mating on tarsus length (r₁₆ = 0.87) which was also correlated with arrival date in both sexes. After removing the variance produced by arrival date, assortative mating remained significant, suggesting that it may have been produced via sexual selection.