Toxicology is about to establish itself as a leading scientific discipline in addressing potential health effects of materials on the nanosize level. Entering into a cutting-edge field, has an impact on identity-building processes within the involved academic fields. In our study, we analyzed the ways in which the entry into the field of nanosciences impacts on the formation of disciplinary identities. Using the methods of qualitative interviews with particle toxicologists in Germany, Holland, Switzerland and the USA, we could demonstrate that currently, toxicology finds itself in a transitional phase. The development of its disciplinary identity is not yet clear. Nearly all of our interview partners stressed the necessity of repositioning toxicology. However, they each suggested different approaches. While one part is already propagandizing the establishment of a new discipline – 'nanotoxicology'- others are more reserved and are demanding a clear separation of traditional and new research areas. In phases of disciplinary new-orientation, research communities do not act consistently. Rather, they establish diverse options. By expanding its disciplinary boundaries, participating in new research fields, while continuing its previous research, and only vaguely defining its topics, toxicology is feeling its way into the new fields without giving up its present self-conception. However, the toxicological research community is also discussing a new disciplinary identity. Within this, toxicology could develop from an auxiliary into a constitutive position, and take over a basic role in the cognitive, institutional and social framing of the nanosciences.