Interphase and mitotic nuclei of Zygorhynchus moelleri were examined by serial sections of cells fixed with a conventional procedure incorporating an enzymatic cell wall degrading pretreatment (CF) and with a freeze substitution process (FS). CF is superior for analysis of nucleus-associated organelles (NAO) and the nuclear envelope. FS is best for microtubules because, during interphase, it alone retains a population of about 10 short intranuclear microtubules and also preserves more NAO-associated cytoplasmic microtubules. However, during mitosis, while FS gives clearer images of spindles, both procedures show the same numbers and types of spindle and cytoplasmic microtubules, thus supporting the use of enzyme pretreatment schedules for studies of fungal spindles. Mitosis characteristically utilizes about 6 presumptive kinetochore microtubules which remain very short (0.04-0.18 mum) and clustered around the spindle poles throughout mitosis. The rest of the spindle initially contains both interdigitating and continuous microtubules which elongate, reduce in numbers, and probably slide apart as the spindle elongates. The variously oriented, NAO-associated cytoplasmic microtubules do not appear to aid in spindle elongation. Chromatin remains dispersed throughout the nuclear cycle. The presence of presumptive kinetochore microtubules clarifies earlier reports on this group of fungi, but their unusual behaviour supports previous suggestions about the primitive nature of the spindle.