The Afrotropical complex of sibling species Anopheles gambiae Giles includes the most efficient vectors of human malaria south of the Sahara. Anopheles arabiensis Patton and An. gambiae s.s. Giles are the members of the complex more adapted to the human environment. They are sympatric and synchronic over most of their distribution range; however, they show a different involvement in malaria transmission, with An. gambiae being more anthropophilic and endophilic than An. arabiensis. Discriminating between them is essential for a correct assessment of epidemiological parameters. The identification is currently achieved through recognition of species-specific chromosomal inversions or by molecular biology techniques. Both methods require considerable technical resources, not always available in the field. We carried out a morphometric analysis of field and laboratory samples of An. arabiensis and An. gambiae s.s. from sites in Madagascar, Burkina Faso, Mali, and Liberia to evaluate the degree of morphological differentiation. We examined 17 morphometric characters in samples representing each of the geographic sites. All of the measures were significantly larger for An. arabiensis (regardless of the collection site), demonstrating an intrinsic greater body size of this species. To assess the reliability associated with the multivariate statistic, we applied the discriminant function analysis, which provided a method for predicting to which group a new case will most likely be assigned. In a blind experiment, the morphometric method correctly identified approximately 85% of field-collected An. arabiensis and An. gambiae s.s., which provided a relatively simple method to approximate the relative frequencies of the 2 species in areas in which their concurrent presence was already known. The influence of laboratory conditions on the morphometrics of the 2 species was also analyzed.