This paper adapts the similarity avoidance analysis developed by Frisch, Pierrehumbert and Broe (2004) for Arabic to account for co-occurrence restrictions in the set of reconstructed verbal roots of Proto-Indo-European (PIE). Completion of the two components of the similarity avoidance methodology – the identification of consonantal co-occurrence restrictions through quantification of over- and under-representation in the data, and the appeal, as a means of explaining them, to values of similarity calculated according to shared natural classes, reveal a picture of co-occurrence noticeably more fine-grained than is conveyed by the individual constraint statements traditionally posited for the language. The analysis also evokes questions about the justification for one of these constraints in particular, that against co-occurrence of voiced unaspirated stops; the issue is examined here in further detail.