The habits and dispersal of the pteromalid parasite Nasonia vitripennis and its housefly host, Musca domestica, and their density relationships were investigated. Male parasite longevity averaged 1.62 days and female longevity 6.96 days. The average number of progeny per female parasite was 139.8. When 25 pairs of parasites were placed with 300 housefly pupae of three sizes, the percentage parasitism was 45.0 for the small (less than 2.0 mm. in diameter), 72.8 for the medium (from 2.0 to 2.5 mm.), and 65.7 for the large (greater than 2.5 mm.). The number of parasites produced per parasitized pupa was 2.20 for the small, 4.30 for the medium, and 6.11 for the large pupae. Increasing the extent of superparasitism produced significantly smaller parasite progeny. A significantly greater normal mortality occurred in small housefly pupae (21.5 per cent) compared with either the medium (6.3 per cent) or the large (5.2 per cent).

The parasite dispersed significantly faster when unfed than when fed. Housefly pupae which had been previously exposed to parasites also stimulated the tendency for subsequent parasite emigration. Housefly dispersal was found to be density-dependent under the experimental conditions of the multi-cell population cage.

With an increasing parasite-host ratio the rate of parasitization increased until all hosts at a 1 to 1 ratio were parasitized in 24 hours; percentage of parastized hosts producing parasites reached a peak of 71 per cent at the 1 to 2 ratio; and above the 10 to 1 ratio all parasite progeny died due to super-parasitism.