Results of qualitative and quantitative studies of prenatal skeletal development in Macaca mulatta are presented. Longitudinal radiographic observations were carried out on 20 monkeys of known gestational age, beginning on 120 days of gestation until the neonatal stage of skeletal development. These studies were based on multiple uterotomies on each pregnant female. The technique described provides accurate data on prenatal bone ossification, and permits an accurate estimation of fetal age in pregnant rhesus monkeys with unknown conception dates.