Using Norwegian survey data from 2003, this paper examines whether cohabiting fathers have a weaker commitment to fathering than do married fathers. Two differences between cohabiting and married fatherhood are thought to affect fathering commitment: the level of institutionalisation; and relationship quality. Three indicators of fathering commitment are studied: the father's willingness to allocate time to his family over work time; his willingness to ensure his child siblings; and to be a good parent to his child. Married fathers are found to be more likely than cohabiting fathers to agree that their child needs siblings. This indicates that even in a country where cohabitation is very marriage-like, the union status of co-resident fathers has some relevance for their commitment to fathering.