In sexual microbes, mating occurs by fusion of individual cells. This complete fitness investment suggests that cell behaviour could potentially mediate prezygotic isolation between microbial species, a topic about which very little is known. To investigate this possibility, we conducted individual cell mate choice trials and mass-culture mating propensity assays with isolates from sympatric natural populations of the closely related yeasts Saccharomyces cerevisiae and Saccharomyces paradoxus. Although we found no evidence for active species recognition in mate choice, we observed a marked difference in mating propensity between these two species. We briefly discuss the possibility that this mating propensity difference may contribute to reproductive isolation between S. cerevisiae and S. paradoxus in nature.