Approximately 20-30% of adult, 60-70 day old, dBA/2J male mice exhibit infanticide toward standard stimulus 1-3 day old Rockland-Swiss Albino mouse pups, while 50-80% of adult C57BL/6J males display the behavior. Reciprocally-crossed hybrid males exhibit DBA-like levels of low pup-killing behavior and cross-fostered DBA and C57BL males retain their strain-typical phenotype. Also DBA males continue to exhibit low levels of infanticide and C57bL males display high levels of the behavior regardless of the length of pup exposure, or the sex, age, or strain of the stimulus newborn. Thus, strain differences in pup-killing behavior are not related to differences in the prenatal or postnatal maternal environment, or to situational or experiential factors. Inherent biological differences may be responsible for the strain differences in infanticidal behavior.