Anatomical measurements, cytological information, and a general physical description of two gibbon (Hylobates muelleri abbotti) X siamang (Symphalangus syndactylus)hybrid apes are presented. The characteristics of the hybrids are compared with those of the parents and other members of the parental species. These hybrids are the only known crosses between genetically distinct ape species. The hybrids displayed a mosaic of gibbon and siamang characteristics. The first hybrid had the coloring and, in general, the more cumbersome body proportions of the siamang. She lacked the siamang throat sac and like the gibbon, developed ischial callosities at an early age. The second hybrid, which died at 5 months, generally resembled the siamang. She had a dark coat, had no face ring, and did not develop ischial callosities. The conclusion of the paper examines genetic influence on morphology and behavior as illustrated by the study of this ape. Finally, the impact of this birth on current cytogenetic and evolutionary theories is discussed. © 1980 Plenum Publishing Corporation.