Numerous insects, including Drosophila nigrospiracula, that inhabit necrotic cacti of the Sonoran desert are host to Macrocheles subbadius, a cosmopolitan mesostigmatid mite. This paper shows that the mite pierces Drosophila integument and ingests haemolymph while attached to the abdomen of its host. Among field—caught flies, there was a strong negative relationship between mite load and wet mass of adult flies of both sexes. Thus, M. subbadius is an ectoparasite of D. nigrospiracula, contrary to the widespread belief that macrochelids form only phoretic associations with adult flies. The parasite harms its host in many ways. Under laboratory conditions, there was a significant negative effect of mite load on survivorship. In the field, there was a significant negative relationship between mite load and number of eggs carried by sexually mature females, suggesting that infested females resorb their oocytes to compensate for nutritional stress imposed by feeding mites. Likewise, in the laboratory, infestation lengthened somewhat the period prior to onset of oviposition, and decreased the number of eggs laid by females over their life. However, mite—infested females maintained on a yeast—supplemented diet overcame the potentially debilitating effects of mites, and were actually slightly, but not significantly, more fertile than uninfested females. Since the dietary conditions of flies influenced the degree to which fecundity was affected, the debilitating effect of mites is comparable to that of starvation. This conclusion derives support from the observation that laboratory females infested for 4 d, but whose mites were subsequently removed, produced fewer progeny than uninfested females. Although this effect was significant only among old females, it nevertheless indicates that the observed effects of mites on fecundity are not the result of mechanical constraints on oviposition behavior. Since intensities of infestation used in laboratory experiments were similar to those encountered in nature, observed ectoparasitic effects on fly fitness suggest that M. subbadius can reduce fly numbers in nature.