Homothallic cultures of Schizosaccharomyces pombe, anaerobically grown to stationary phase in broth at 32 degrees C, were induced by aeration to flocculate. Flocculation was followed by copulation, conjugation, zygote formation, meiosis and sporulation. Cultures grown to stationary phase at 32 degrees C and then aerated at 37 degrees C did not sporulate. Grown to stationary phase at 37 degrees C, cultures were not immediately inducible when aerated at 32 degrees C. To identify which events in the developmental sequence were thermosensitive, we grew and induced cultures at 32 degrees C and then shifted them at various times to 37 degrees C. We observed the following events to be thermosensitive: development of respiratory sufficiency, readiness (inducibility of a culture within 1 h), flocculation induction, copulation, conjugation and early sporulation (including meiosis). Respiration, flocculation and spore maturation were thermoresistant. Conjugation-induced lysis and postdevelopmental deflocculation were enhanced at 37 degrees C.