The fine structure of ascospore formation in the fission yeasts Schizosaccharomyces pombe and Schizosaccharomyces japonicus var. japonicus was studied by serial thin-sectioning and electron microscopy. The morphogenetic events were almost the same in both species. Ascospore development was initiated by the formation of the forespore membrane on the cytoplasmic side of the differentiated nucleus-associated organelle (NAO) in the interval between meiosis I and II in S. pombe, or during the post-meiotic nuclear division in S. japonicus, and the process proceeded almost synchronously through the two or four nuclei in the ascus. The forespore membrane developed by fusion of the cytoplasmic vesicles and this was clearly demonstrated in S. japonicus where the behaviour of vesicles involved in the forespore membrane development could be traced as they were marked by the presence of electron-dense granules. The staining technique, by phosphotungustic acid--chromic acid (PTA-CA) after treatment with periodic acid, was used to attempt to elucidate the origin and the nature of the forespore membrane. The method specific to plasmalemma-type membranes stained both ascus and ascospore plasmalemmas; the forespore membrane was not stained at first but developed the same affinity for stain as the plasma membrane in the course of ascospore development. The results suggest that the forespore membrane did not come directly from the ascus plasma membrane, but from another membrane system such as the endoplasmic reticulum. Spore wall material was deposited in the space between the inner and outer leaflets of the forespore membrane.