In Kathmandu valley, two populations of rhesus monkeys which are totally protected, have shown relatively stable numbers over a period of several years. Population stability within heterosexual troops appears to have been maintained through lower birth rates and slightly higher infant and adult mortality rates than in comparable rhesus populations in India which have been subject to trapping. Although the behavioral and physiological mechanisms by which these demographic changes occur are not known, behavioral observations on these populations suggest several possibilities. These data represent the first indication of possible mechanisms for population regulation in natural rhesus populations.