ABSTRACT The effect of altered tyrosyl-tRNAs on the developmental process of sporulation was examined. Mutations in eight independent loci resulting in tyrosine-inserting nonsense suppressors were tested for their effects on sporulation. Different levels of inhibition were found ranging from SUP3-o, which caused the greatest reduction of sporulation (7-17% of wild type), to SUP11-o which caused no reduction in sporulation. Since the SUP3-o mutation exhibited the greatest effect, it was studied in detail. Although SUP3-o is a dominant nonsense suppressor, its effect on sporulation is recessive. Expression of the sporulation deficiency is dependent upon the stage of transfer from glucose growth medium (i.e., log, early stationary, etc.) to sporulation medium. SUP3-o/SUP3-o diploid cells transferred from log or early stationary phase are capable of sporulation, whereas cells transferred after early stationary phase (i.e., after adaptation to respiration) exhibit poor sporulative ability. Sporulation events were examined under restrictive conditions to observe those events completed by SUP3-o/SUP3-o diploids. The early events of sporulation occur in these cells. Later events are completed by progressively fewer cells. Premeiotic DNA synthesis occurred in approximately 40% of the cells, nuclear segregation occurred in 20%, and finally, only 2% formed asci. The fact that fewer late-sporulation events occur under restrictive conditions can be explained by increased efficiency of suppression.