This article investigates whether linkages between members of parliament (MPs) and interest groups matter for MPs' activities of co-sponsoring legislative proposals. Based on statistical models for network data, the study builds on classical explanations of co-sponsorships highlighting the role of similar ties between MPs, such as party membership, legislative committee assignments, electoral district or gender. It shows that, on top of these traditional forms of homophily, MPs' ties to interest groups also make a difference. MPs with ties to a similar type of interest groups are more likely to co-sponsor their respective proposals. The same holds for MPs with ties to groups active in the same policy domain. These findings have implications for the study of groups' lobbying, legislative behaviour and representative democracy.