Female mating preferences can lead to the evolution not only of exaggeration in the preferred character, but also of sexual dimorphism. In seaweed flies (Coelopa frigida) most females exhibit a preference for large males, although the strength of the preference is known to vary in different populations. In this study the extent of sexual dimorphism in adult size was determined in samples from 30 widely distributed natural populations. The mean dimorphism in a population was found to be strongly correlated with mean male size; however, this relation depended on the $\alpha \beta $ inversion karyotype which is a major genetic determinant of adult size. The $\alpha \alpha $ males were usually larger than $\alpha \alpha $ females, but in some populations the reverse was found for $\beta \beta $ karyotypes. The evolution of this pattern of sexual dimorphism is discussed in the context of female mating preferences. It appears that the exaggeration of the male character due to sexual selection has only occurred on the $\alpha $ form of the inversion; the $\beta \beta $ males appear to have remained at their optimum size as determined by natural selection.