Abstract Twenty-two strains of viruses, each possessing adenovirus complement-fixing antigen and each derived from fecal specimens of chimpanzees, have been classified by their hemagglutinating properties into three major subgroups. Subgroup I is composed of eight strains which cause hemagglutination of rhesus or vervet monkey erythrocytes; subgroup II consists of 12 strains which agglutinate selected Sprague-Dawley rat erythrocytes in the presence of heterotypic immune serum to members of the Rosen subgroup III of human adenoviruses; and finally, chimpanzee subgroup III is composed of two strains which fail to agglutinate monkey, rat, guinea pig, or human type O erythrocytes. HAI reciprocal tests suggest that all eight of the subgroup I strains are related to prototype C-1 chimpanzee adenovirus, as well as to human adenovirus type 16. Six strains of subgroup II are closely antigenically related to each other and to human adenovirus type 5 by HAI. Four additional strains are partially cross-reactive in HAI tests with human adenovirus type 2 and highly cross-reactive with one another. The remaining two strains of subgroup II apparently represent previously unreported serotypes without relationships to known adenoviruses or to each other, as demonstrable by HAI techniques. Evidence is presented that the 22 strains represent a total of at least seven distinct chimpanzee serotypes of adenovirus.