Careers of early career researchers (ECRs) are significantly determined by the structural conditions within a scientific system. Due to the low predictability of career prospects in and outside academia, early and ongoing proactive career behaviour is crucial for their career development. However, ECRs' engagement in proactive career behaviour has not been sufficiently considered yet. Based on the Social Cognitive Model of Career Self-Management (Lent and Brown 2013), this study examines the extent of ECRs' engagement in career-related activities and favourable influences. Data of 1,186 research associates at a German and a Swiss university were collected in 2018. Descriptive analyses reveal that ECRs are more likely to carry out career-related activities which are proximal to the scientific profession and qualification (e.g. scientific collaboration) than those that involve a higher degree of self-initiative (e.g. participating in career counselling). Both individual characteristics and factors of opportunity structures like support in the work environment stimulate proactive career behaviour. Effects are partly moderated by the level of formal qualification. A general barrier is a low or moderate interconnectedness at the university. Starting points for promoting ECRs are discussed.