We measured the relation between search rate ofMicroplitis croceipes(Cresson) and density of third instars of its hosts,Heliothis zea(Boddie) andHeliothis virescens(F.). We released wasps in field cages containing various densities of host larvae on wild host plants (chieflyGeranium dissectumL.) in spring and on cotton (Gossypium hirsutumL.) in summer. Number ofHeliothisspp. larvae parasitized increased linearly over the range of host densities used. Such responses may be more common than has recently been assumed. A simple search model first proposed by Nicholson&Bailey (1935) fit the data well. Search rate was 0.20 ± 0.03 m2per day (mean ± SE) in spring and 0.55 ± 0.04 to 0.87 ± 0.09 m2per day in summer. Neither air temperature nor host-plant size explained these differences completely.