Animals foraging for patchily distributed resources may optimize their foraging decisions concerning the patches they encounter, provided that they base these decisions on reliable information about the profitability of the habitat as a whole. Females of the parasitoid Lysiphlebus testaceipes exploit aphid hosts, which typically aggregate in discrete colonies. We show here how between-colony, travel time and the number of aphids in previously visited colonies affect parasitoid foraging behavior. We first assumed that parasitoids use travel time and previous colony size to estimate a mean rate of fitness gain in the habitat and derived quantitative predictions concerning the effect of these two variables on patch residence time and patch-leaving rate of attack. We then tested these theoretical predictions in laboratory experiments in which female parasitoids were allowed to visit two successive colonies. As predicted, the observed residence time in the second colony increased with increasing travel time and decreasing size of the first colony. Patch-leaving rate of attack decreased with increasing travel time but was not affected by previous colony size. These results suggest that parasitoids use these two variables to assess habitat quality. However, discrepancies between the data obtained and quantitative predictions show that the effect of travel time on patch use may be more complex than assumed in our model.