The effect of biological motherhood on parents' transition to parenthood and their division of labor is a much contested issue. This paper explores the impact of biological motherhood in a unique context—lesbian parenthood—where biological requirements can be analytically separated from gender effects. The analysis is based on a study of 25 middle-class lesbian couples' transition to parenthood and their division of labor. Each couple had at least one biological child under the age of six and all children were born within the context of the couples' relationship. I conducted in-depth interviews with each partner and all participants filled out a short questionnaire. The distinction between the biological and non-biological mother affected couples in three domains of motherhood: public, relational, and personal motherhood. Comothers countered public ignorance, social and legal invisibility, and the lack of biological connection to the child by sharing primary childcare and establishing a distinct parenting role within the family. The participants employed various models of the division of labor to provide one full-time mother for as long as economically possible. Desire to be with the child, economic considerations, and strong commitments to equality and shared motherhood rather than biological requirements informed decisions about leave strategies and long-term paid work arrangements. Time/availability proved to be the best predictor of involvement in family work. Conflicts erupted whenever one partner perceived the other as not doing her fair share of domestic work.