AbstractObservations on oppositely sexed pairs ofM. fascicularisandM. mulattarevealed certain differences.M. fascicularisshowed single‐mount copulations, higher potency, less grooming activity, and milder but more frequent aggressive exchanges than the rhesus monkey. These differences might be due to the lower dominance gradient between the sexes and less sexual dimorphism inM. fascicularisthan inM. mulatta. There were changes in the sexual, grooming, and aggressive interaction in relation to the menstrual cycle, indicating the suitability ofM. fascicularisfor hormone‐behavior studies. Comparison of data from 30 and 60 min tests showed that important behavioral changes did not become apparent unless animals were given enough time to inter