An hfd1-1 mutant of Saccharomyces cerevisiae, SOS4, characterized by predominant production of two-spored asci at 29 degrees C, undergoes normal meiotic nuclear divisions and produces four haploid nuclei, but only two non-sister nuclei among them are incorporated into mature ascospores. Spindle pole bodies and prospore wall membranes at the second meiotic division at 29 degrees C were observed in this mutant by electron microscopy. The spindle pole body at one pole of each spindle had a normal outer plaque which was larger than the inner plaque. At the other pole, the outer plaque was entirely absent and a normal prospore wall membrane was not detected. It was concluded that at 29 degrees C the hfd1-1 mutation blocks the morphogenesis of outer plaques and prospore wall membranes at the two non-sister poles in the second meiotic division, and consequently only non-sister nuclei in the resulting meiotic cell are incorporated into ascospores.