A field experiment addressed four questions concerning the interaction betweenMicroplitis croceipes(Cresson) andHeliothisspp.: (1) what is the relation between the proportion of hosts parasitized and host density? (2) does this relation depend on the size of the areas (spatial scale) at which it is measured? (3) what is the relation between parasitoid search rate and host density? and (4) what is the relation between parasitoid distribution and host distribution? To answer these questions a cotton field was artificially infested with various densities ofHeliothis virescens(F.).M. croceipeswas released uniformly across the field. At the scale of 30 by 60 m plots, the proportion of hosts parasitized byM. croceipes(adjusted for competition with other parasitoid species) increased from 0.41 ± 0.08 (x± SE) to 0.60 ± 0.06 as host density increased from 6,178 to 16,896 larvae/ha. The calculated search rate (area searched per parasitoid per unit time) ofM. croceipeswas independent of host density, and the density dependence in parasitism byM. croceipescould be explained completely by density dependence in adult female distribution. Natural populations ofCardiochiles nigricepsViereck andCotesia marginiventris(Cresson) parasitized 9 and 7%, respectively, of the hosts collected. At the scale of plots, parasitism byC. nigricepsincreased with host density, but parasitism byC. marginiventriswas independent of host density. At the scale of 1 by 2 m quadrats, parasitism by all species was independent of host density. We discuss the implications of these results for biological control ofHeliothis virescensandHelicoverpa zeaand for theories of parasitoid foraging and population regulation.