Despite an extensive literature on inbreeding depression in a variety of traits, relatively little is known about the effects of inbreeding on patterns of sexual behaviour. In this study, we use the guppy Poecilia reticulata to investigate the influence of parental relatedness (full-sib. vs. unrelated matings) on sexual behaviour and colour pattern variation in adult male offspring. Our results revealed that one generation of full-sibling mating resulted in a strong decline in male sexual motivation (courtship intensity and following behaviour) and mating success (number of successful copulations and insemination success - as estimated by the frequency of post-copulatory jerks after mating), but we detected no significant influence of inbreeding on colour pattern variation. Our finding that sexual ornamentation (colour) did not show similar inbreeding depression to courtship behaviour may be due to differences in the genetic basis of these traits. Our findings indicate that courtship can be a sensitive indicator of genetic stress and add to an emerging picture that sexually selected traits can be severely affected by inbreeding depression.