We demonstrate that during the courtship stage of conjugation, S. cerevisiae a cells choose the alpha cell producing the highest level of pheromone from among potential mating partners. From this result and that for alpha cells we conclude that both a and alpha cells act as signaling cells during courtship, that both cell types respond by discriminating different levels of signal, and that the signals are the mating pheromones. Responding cells that are supersensitive to signal fail to discriminate pheromone-producing from nonproducing cells to an extent that depends on their degree of supersensitivity. We propose that partner selection in S. cerevisiae results from polarized morphogenesis of a responding cell in the direction of highest pheromone concentration and that cells defective in discriminating this gradient execute a default pathway in which an adjacent cell is selected at random.