Falling in-between the category of employers and employees, solo self-employed persons are poorly accommodated by the traditional system of corporatist interest representation through which the interests of employers are represented in employers’ associations, and the interests of employees are represented by trade unions. For business associations ‘micro businesses’ with no employees are difficult to reach and serve, as their interests are considered distinct from companies with employees. Trade unions in many countries even resisted the emergence of solo self-employment, fearing that these and other flexible arrangements would undermine not only secure working conditions but also union solidarity. This study examines differences in membership of interest organizations among solo self-employed workers. Various types of organizations are investigated, not only trade unions and business associations, but also new freelancer associations. Theoretically, three categories of determinants are examined to explain differences in membership: economic explanations, social explanations, and political explanations. Empirically, survey data are used from the Netherlands, a country with an increase in self-employment and a diverse supply of interest organizations. It is concluded that economic determinants explain both ‘generic’ membership (of any organization), as well as membership of specific organizations. Social and political determinants do not explain generic membership, but instead help to explain why some self-employed workers join particular types of organizations.