In the stream-dwelling isopod Lirceus fontinalis, males and females engage in a precopulatory mate guarding phase prior to mating. We examined the energetic costs of mate guarding behavior in males by separately assaying glycogen and lipid content at different time increments following mating. We found that males that had recently mated possessed reduced glycogen reserves and that these reserves were fully replenished within 36 h. Conversely, we found that male lipid reserves were unaffected by time since mating. We concluded that precopulatory mate guarding behavior is energetically costly to males and that glycogen is the energy source utilized to pay that cost. We also examined whether food deprivation during the mate guarding phase affected male energy reserves (glycogen) at the end of that phase. We found that males that were held in the laboratory and starved during mate guarding possessed reduced glycogen at the termination of the phase when compared to fed males. This reduced quantity was equivalent to the glycogen reserves of recently mated males collected from the field. We propose that food deprivation during the mate guarding phase explains the reduction in glycogen reserves at the termination of that phase. We discuss these results with reference to patterns of refuge use behavior during the mate guarding phase.