Experiments were conducted to determine whether previous encounters with host spiders would affect hunting behavior in females of the spider wasp, Pepsis mildei. Each wasp was subjected to 3 encounters with a different spider (Aphonopelma steindachneri) in an experimental chamber. The mean time required for wasps to orient their bodies and approach the host decreased significantly by the second encounter (546.1 sec ± 15.2 SD) as compared to that required by naive wasps on their first encounter (972.4 sec ± 19.3). In a similar fashion, mean time required to paralyze the host decreased from 198.5 sec ± 13.3 SD on the first encounter to 156.7 sec ± 10.2 by the second encounter. The mean number of movements of the antennae by these wasps for various time intervals during the hunting sequence (interval between the initial orientation of wasp toward the spider, and first contact; and between last bout of antennation of host and insertion of stinger) also decreased significantly between the first and second encounters, suggesting that the time required to assess the suitability of a host decreased as a function of experience as well. These results indicate that the hunting behavior of pepsine wasps is characterized by a certain degree of plasticity that may contribute to the survivorship of these wasps. This plasticity may be due learning processes or timing-events associated with sensitization.