The size and asymmetry of male secondary sexual traits are believed to convey reliable information to females concerning the quality of potential mates. Experimental manipulations of male sexual traits provide a powerful approach to the study of preference evolution. Nevertheless, the majority of studies rely on correlational evidence for selection acting on secondary sexual traits. Here we report that in three of four populations of the European earwig, Forficula auricularia, females mated sooner with males with longer forceps although there was no female preference based on forceps asymmetry. We isolated the potentially confounding influences of forceps length and asymmetry by independently manipulating each trait. Manipulations of forceps length confirmed that females preferred males with longer forceps. However, manipulations of asymmetry revealed that although females pay attention to forceps condition, they show no preferences based on asymmetry. No relationships were found between the length and asymmetry of forceps in field populations, and there were no differences in condition between symmetrical and asymmetrical males. Our results are consistent with the notion that female choice has contributed to the sexual dimorphism in earwig forceps. However, they refute the notion that fluctuating asymmetry plays a role in sexual selection. Copyright 1998 The Association for the Study of Animal Behaviour