AbstractIn an apomitic strain ofSaccharomyces cerevisiae(ATCC 4117‐H2) which undergoes a single nuclear division during sporulation and consequently forms asci containing two uninucleate diploid spores, a study was undertaken to investigate the effects of cultivation in three presporulation media (YPA; YNB; SMM) on nuclear division and ascoporogenesis in sporulation medium. Comparison of effects of presporulation culture in these media on the number of spores formed per ascus showed that a marked induction (30 ± 4·3 per cent) of three‐ and four‐spored asci could occur in sporulation medium following cultivation in a defined YNB medium supplemented with a 1 per cent solution of vitamins and containing decreased ammonium sulphate and increased glucose levels. Experiments in which the concentrations of glucose and of ammonium sulphate were varied simultaneously indicated that the initial presporulation carbon to nitrogen source ratio is an important factor in determining tetrad formation in sporulation medium. Nuclear staining demonstrated two classes of asci: binucleate (one‐ and two‐spored) and tetranucleate (three‐ and four‐spored). Genetic evidence and data concerning effects of inclusion in sporulation medium of a meiotic inhibitor (glucose) indicated spores in tetrads were haploid rather than diploid. This ability to condition a significant number of cells for meiotic rather than apomictic differentiation made possible investigation of effects of mitochondrial inhibitors on both developmental processes simultaneously. It was found possible to selectively inhibit meiotic development by inclusion in sporulation medium of appropriate concentrations of specific inhibitors. Moreover, the data suggest meiotic sporulation is more strictly dependent than apomictic sporulation on mitocho