This study seeks to establish the effect of parliamentary specialisation on cosponsorship of parliamentary proposals in parliamentary systems with high levels of party unity. Existing studies on presidential systems suggest that cosponsorship is mainly related to legislators’ policy preferences. It is proposed that in parliamentary systems cosponsorship is, in the first place, structured by the division of labour in parliamentary party groups: MPs who do not have overlapping policy portfolios will not cosponsor proposals. Other explanations, such as policy distance and the government–opposition divide, only come into play when MPs are specialised in the same field. This expectation is tested using data from the Netherlands, a parliamentary system with a clear division of labour between MPs. It is found that specialisation has a very large impact on cosponsorship.