The present study explores how perceived demographic and attitudinal similarity can influence proactive behavior among organizational newcomers. We propose that newcomers who perceive themselves as similar to their co-workers will be more willing to seek new information or build relationships, which in turn will lead to better long-term adjustment. Results from a three-wave field investigation of newcomer proactive behavior show that newcomer perceptions of surface-level similarity to the rest of the work group in education and gender increased proactive adjustment behavior. Contrary to our expectations, perceived similarity in terms of age decreased proactive adjustment behavior--in other words, newcomers who were significantly different from their co-workers in age engaged in more proactive adjustment behaviors. Deep-level similarity in terms of work style was associated with higher levels of role clarity, but this relationship was not mediated by proactive adjustment behavior. (Contains 4 tables and 1 figure.)