Nutritional conditions regulate mating of the fission yeast S. pombe. To investigate how nutritional signals are monitored by the cell and translated into appropriate mating behaviour, effects of unique and specific growth factors would be desirable. We show that thiamin can inhibit sexual agglutination and zygote formation in S. pombe. A concentration of 50 nM thiamin in the culture medium is required for full growth of a thiamin auxotrophic strain. At this concentration thiamin starts to inhibit mating of wild-type cells of opposite heterothallic mating type and at a 1 microM concentration zygote formation is inhibited by more than 95%. Growth conditions modulate the inhibitory effect of thiamin. Thiamin acts only for a restricted period of time and seems to inhibit commitment to zygote formation rather than the cell aggregation and fusion process itself. Pyrithiamin, a thiamin antagonist, inhibits growth as well as mating.