The functional response of the parasiteCotesia flavipes(Cameron) was measured at low densities of the hostDiatraea saccharalis(F.) in field cages containing corn plants. Hosts were exposed to parasites by allowing host larvae to bore into the corn stalks and create the natural microhabitat associated with parasite foraging. Of 992 hosts exposed, 785 were recovered, of which 689 produced adult moths. In total, 96 hosts were considered attacked byC. flavipes. Thirty-four (35.4%) of the attacked hosts were successfully parasitized and producedC. flavipesprogeny, and an additional 62 attacked hosts (65.6%) did not pupate or produce parasites, as a result of encapsulation of parasite progeny by the hosts. The 34 successfully parasitized hosts produced an average of 41.2 parasite progeny per host, of which 29.3 were female, giving a sex ratio (females-males) of 2.33: 1. Use of linear and nonlinear regression to describe the functional response gave nearly equal predictions of attack rates and fit the data equally well. The functional response was indistinguishable from a linear response over the range of host densities tested, whether parasitization was defined as the number of hosts producing parasite progeny, or the total number of hosts attacked. The two different measurements of the functional response ofC. flavipesare important for estimating the success of parasite search inferred from the data, because attack rates that include only the number producing parasite progeny greatly underestimate the number of parasites finding hosts. The near linearity of the functional response is likely due to measurement of parasite success at realistic host densities, which avoids the limitations caused by measurement at artificially high host densities.