Desiccation resistance and water balance were studied in the adult female mosquitoes Anopheles arabiensis and Anopheles gambiae sensu stricto. When the two species were reared from egg to adult under identical conditions, An. arabiensis had significantly higher desiccation resistance than did An. gambiae. Data are presented that indicate that this difference in desiccation resistance is associated with a higher body water content prior to desiccation in An. arabiensis. No differences in rate of water loss during desiccation or water content at death were observed. Measurements of metabolic rate and respiratory pattern also showed no statistically significant differences between the species. This study provides the first physiologic measurements of desiccation resistance in adults of these species and offers insights into the physiologic differences associated with differential resistance to desiccation stress.