We used feathers and skeletal measurements, white and black areas at the wing tip and bill morphometrics of Yellow-legged Gulls (Larus michahellis lusitanius) in order to test for possible sex-differences. Overall, 157 individuals from the eastern Bay of Biscay (N Spain) were measured, and the sex determined in 155 individuals, by means of DNA-analyses. All feathers and skeletal-associated measurements, except the distance between each primary (P1 to P5) feather and the wing tip in a folded wing varied between the sexes, with males being larger than females. Sexual selection is discussed to be the major cause explaining these differences. A discriminant function is provided to separate sexes. By contrast, both wing tip patterns of coloration and bill morphology did not vary between sex classes.