A cooperative biological control project against the pink hibiscus mealybug, Maconellicoccus hirsutus, infestation in the southern, low-desert region of California was initiated in the fall of 1999. Subsequently, two encyrtid parasitoid species were mass reared and released. This was followed by the release of a third species, Allotropa sp. nr. mecrida, in the family Platygastridae. Population densities of mealybug and percent parasitism were monitored monthly at five mulberry tree sites and three carob tree sites. The population density within the first year was reduced by more than 95%. This was primarily a result of the parasitoid Anagyrus kamali. Gyranusoidea indica was rarely found during much of each year, however it did represent as much as 20% of the parasitoid species composition during mid to late fall. Over the entire four-year period of the project, the average regional population density of the mealybug exhibited a continued decline. Hyperparasitism of Anagyrus kamali was frequently over 35% during 2000. A density dependent relationship occurred over subsequent years, whereby hyperparasitism was considerably lower during each successive year, coincident with declining numbers of both mealybug and the primary parasitoid host. Field collections of two, common non-target resident species of mealybugs suggest that they are not being utilized as alternate non-target hosts by the newly introduced parasitoids.