Abstract.1. The relationship between the parasiteMuscidifurax raptorand its host, puparia of the Mediterranean fruit flyCeratitis capitatawas studied. Emphasis was placed on quantitative descriptions of parasite responses and behaviour.2. Female parasites ‘paralyse’ the host prior to oviposition. This treatment is not essential for larval development but crucial when oviposition took place in fully developed pupae. When having the choice females preferred to parasitize older hosts.3. Parasite efficiency was affected by temperature, and reached its greatest value at 28°C. Increasing parasite density reduced the searching efficiency when it exceeded a certain level. A stronger effect of similar type was obtained by increasing the number of males per female. Apart from affecting efficiency of search, increasing parasite density caused an increasing proportion of males in the progeny.4. Parasites responded to host density by weak functional response followed by reduction in efficiency of search due to egg limitation.5. The parasites responded to host distribution by aggregating on high host densities. Such aggregation is antagonistic to the tendency to disperse due to mutual interfer