In a colony of squirrel monkeys (Saimiri sciureus) of Brazilian origin, 40% of all first pregnancies occurred among females aged 3.5–4 years; 57% of all first pregnancies occurred between the ages of 3.5 and 5 years while 62% occurred between the ages of 2.5 and 4 years. A total of 79% of all first pregnancies occurred among females between the ages of 2.5 and 5 years. 70% of all first pregnancies and 67% of all second pregnancies were unsuccessful. Females with unsuccessful first pregnancies were those most likely to have an unsuccessful second pregnancy, and vice versa. The most common reproductive event in the season immediately following first pregnancy was a failure to become pregnant (33%