The sexual behavior of three inbred strains of mice was studied. Sixteen different behavioral measures were scored; twelve of these sixteen measures revealed statistically significant behavioral differences. The results were as follows: I. C57BL males are characterized by speed. They gain intromission sooner, spend less time between intromissions, and ejaculate sooner than do the other two strains. 2. DBA/2 males show significantly longer intromission latencies, interintromissior. intervals, preintromission mount durations, and mounts than do the C57BL and BALB/c males. DBA/2 males ejaculate with fewer intromissions and thrusts than the other two strains. 3. BALB/c males require the longest time (over one hour), the greatest number of intromissions, and the greatest number of thrusts prior to ejaculation. The sexual behavior of the inbred strains of mice was compared to the sexual behavior of the rat, hamster, and guinea pig. Mention was made of the possibility for genitical and physiological studies of the behavioral differences.