Three laboratory experiments were designed to elucidate some of the stimuli that are involved in host micro-habitat searching, the stimulation of host-searching behaviour and in host location by Diachasma alloeum. Visual orientation was found to play an important role in the location of fruits (Crataegus mollis) by D. alloeum. Equal numbers of landings were obtained on host-infested and uninfested haws. A description is presented of the host-searching behaviour by D. alloeum on haws. It was found that probing activity and duration of stay on the fruit are strongly influenced by the presence of a Rhagoletis pomonella larva in the fruit. A quantification was made of the accuracy with which D. alloeum probes the fruit in search of hosts. It is concluded that host movement is the prime stimulus for the location of hosts. Females on haws containing a moving larva exhibited non-random probing. It was found that parasitization of the host by D. alloeum is preceeded by paralyzation of the host. When paralyzed larvae were offered in the same set-up, D. alloeum exhibited random probing.