Field experiments and surveys were conducted in 2003 and 2004 to evaluate the efficacy of using the parasitoid Microplitis mediator (Haliday) to control populations of Helicoverpa armigera (Hübner) in cotton fields in Northwestern China’s Xinjiang Province. The population levels of the second generation of H. armigera in southern Xinjiang in 2003 and 2004 were ≈20–60% above the economic threshold of this pest. The longevity of field-released female and male parasitoids was 7.6 ± 0.4 and 3.9 ± 0.4 d, respectively. Field tests showed that the number of parasitized H. armigera increased with an increasing number of M. mediator cocoons released in the field. The release of 2,250 or 15,000 laboratory-reared M. mediator cocoons per hectare in cotton fields resulted in 38–61% parasitism of H. armigera in 2003 and 35–66% parasitism in 2004. The efficiency of the parasitoids varied considerably with different numbers of parasitoid cocoons and with the frequency and sites of release. However, the highest damage reduction in these studies was obtained with a single release of cocoons and the highest number of release sites. Large-area releases of M. mediator in cotton fields of Shufu, Shule, and Shache counties resulted in >60% parasitism and an 80% decrease in cotton boll and bud damage compared with the control fields. The above results indicated that field release of M. mediator could be used effectively in managing H. armigera in cotton in Xinjiang.