Three zygomycetes (order Mucorales), isolated from clinical specimens, whose generic or species identification were uncertain, were definitively identified on the basis of the production of zygospores resulting from mating studies. These three isolates were identified as Mucor circinelloides f. circinelloides, Rhizomucor pusillus, and Absidia corymbifera. The production of true zygospores, the "last word" in zygomycete taxonomy, should be considered as a diagnostic tool for the definitive identification of rare, unusual, or atypical heterothallic zygomycetes. Practical considerations, however, limit this method to reference laboratories.