In fungi, sexual reproduction primarily depends on the interaction between peptide pheromones and their receptors. Most ascomycete fungi produce two classes of peptide mating pheromones, a simple peptide and a modified peptide. These peptides are recognized by their corresponding receptors on the surface of cells of the opposite mating type to induce the mating reaction. Pheromone diversification may be associated with reproductive isolation, which restricts gene flow among populations; thus, it remains unclear how pheromones diversify without loss of successful mating. Here, I provide a brief review of recent findings on the 'asymmetric' diversification of peptide pheromones in the fission yeast Schizosaccharomyces pombe, and discuss evolution of the mating pheromones in fission yeast.