Females are often subjected to unwanted mating advances from males. Such advances can be costly to both parties. The short-term costs of harassment to females have been widely explored in the literature; however, few studies have measured the direct fitness costs. Moreover, male costs are seldom considered. Conventional wisdom would lead us to hypothesise that sexual harassment is costly; thus, when males and females are housed together, harassment should reduce foraging, growth and reproductive output and may disrupt social interactions. This study quantified harassment costs in both sexes by observing behavioural responses and long-term effects of unsolicited mating in a controlled setting. Sexually mature guppies were subjected to two housing treatments: equal sex ratios or single-sex groups. The effects of male harassment on males and females were assessed by measuring behaviour, growth rate and the number of offspring produced over a period of 6 mo. Contrary to our expectations, our results indicated no significant differences in foraging and growth rates between mixed- and single-sex shoals for either sex. Moreover, there was no significant difference in fry production between mixed- and all-female shoals. Large males showed higher mortality when housed with females. Both sexes showed a reduction in shoaling when in mixed-sex groups. Thus, there appear to be few direct costs of harassment for females in natural, mixed-sex shoals, but males appear to bear significant harassment costs. The study provides insights into reproductive behaviour and life-history traits.