Variation in size-assortative pairing was studied in relation to sexual selection on male body size in the amphipod Gammarus pulex, using both transverse and longitudinal surveys of natural populations. In addition, the influence of male–male competition on size-assortative pairing was tested in the laboratory. In both surveys, the intensity of sexual selection was positive and significant for male body size but not for females, and size-assortative pairing was positive and significant. The magnitude of size-assortative pairing, however, varied significantly between populations. The magnitude of size-assortative pairing was positively correlated with the intensity of sexual selection on male body size in both surveys. In the laboratory, the strength of size-assortative pairing decreased with decreasing male:female ratio. Taken together our results point to a role of interference competition between males in promoting patterns of size-assortative pairing in G. pulex, although alternative explanations (i.e. female resistance) cannot be excluded.