In sexual reproduction one sex can increase its reproductive success at the cost of the other, a situation known as intersexual conflict. In the marine isopod Idotea baltica, males guard females before copulation. The guarding phase is preceded by struggles as females resist males’ attempts to initiate guarding. We determined whether the struggle and/or mate-guarding result in fitness costs in the form of decreasing fecundity and lower levels of the energy storage compounds, glycogen and lipids. Females that underwent the period of struggles with males had decreased glycogen levels compared with females maintained alone. No such cost was found for males. Females guarded by a male also had smaller eggs than females that were not guarded. Thus the intersexual conflict, imposed by the fitness maximization strategy of the males, gave rise to both a fecundity cost and an energetic cost for females. The fecundity cost confirms the existence of intersexual conflict in I. baltica. This cost is shared by males, suggesting that the intersexual conflict restrains the reproductive output of both sexes.