We studied collaborating interface designers and software developers engaged in multidisciplinary software creation work. Twenty-one designers and developers in 8 organizations were interviewed to understand how each specialist viewed team interactions. We also shadowed most participants as they worked on novel software projects with user interface design challenges. A grounded theory analysis of interview transcripts showed that designers and developers construct unique identities in the process of collaborating that provide meaning to their artefact-mediated interactions, and that help them to effectively accomplish the work of creating novel software. Our model of interactional identities specifies a number of aspects of joint project work in which an interactional identity is expressed. We suggest these identities are constructed to bridge a gap between how designers and developers were taught to enact their roles and the demands of project-specific work.