This study explores the importance that guarding duration has on the reproductive success of a male competing with other males for females. In the experiments, single irradiated males were tested in groups of four fertile males and five females, or single fertile males were tested in groups of four irradiated males and five females. Percentages of non-developing and developing eggs, respectively, laid by the females over 2 days was used to determine the relative reproductive success of the focal male. The males were tested in three situations: 1. Guarding duration interrupted by experimenter; 2. Guarding of natural duration; 3. Prolonged guarding (male glued to the female after copulation). The reproductive success of irradiated focal males increased as the guarding duration increased. No such analysis could be done for fertile focal males because they differed among treatments in both guarding duration (manipulated) and interval between consecutive matings. In treatments with prolonged guarding, male reproductive success tended to be higher, but not significantly so. Prolonged guarding decreased variability in reproductive success of fertile focal males. The results showed that guarding is a paternity assurance strategy and suggested that there is a conflict between sexes over guarding duration.