Seventeen percent of the women who had stepfathers in their childhoods had been sexually abused, compared to only 2 percent of those having biological fathers in their childhoods. When a distinction was made between 'very serious sexual abuse,' including penile-vaginal penetration and nonforceful attempted fellatio, cunnilingus, and anal intercourse, and less serious sexual abuse, 47 percent of incest cases involving stepfathers involved 'very serious' abuse stepfathers involved 'very serious' abuse compared with 26 percent involving biological fathers. The higher prevalence and seriousness of incest by stepfathers may be due to low inhibitions against incest among stepfathers and the absence of a father-daughter bond compared to the inhibitions against incest in biological fathers and the strength of the father-daughter bond. These findings suggest that single mothers with young daughters should be particularly careful in selecting mates and in the cultivation of a close relationship with their daughters that will reduce their vulnerability to incest. 3 tables and 17 references.