none Responses of experienced Microplitis croceipes Cresson females in a wind tunnel choice situation were tested at three periods. Training consisted of exposing females to cotton or soybean leaf material with host (Heliothis virescens) larvae for parasitization or without host larvae. Control parasitoids received no initial exposure. Testing periods were: within 1 hour after exposure; 3 days after exposure or after exposure to reverse training. Pairwise comparison of choices of trained wasps versus controls showed a significant difference in only one case. Pairwise comparison of cotton versus soybean exposed wasps within each training level showed significance for "rewarded" wasps in periods 1 and 3 with highest numbers of wasps flying to the source-plant to initial training. Results suggest the occurrence of associative learning of M. croceipes on host-host plants, and indicate a time dependent component in the learning process and irreversibility of this conditioning.