We have isolated a heat-sensitive beta-tubulin mutation, benA33, that blocks nuclear division and nuclear movement at restrictive temperature. This blockage demonstrates that the beta tubulin encoded by the benA gene is essential to both processes. The blockage of both processes is suppressed by the alpha-tubulin mutation, tubA1; thus the alpha tubulin encoded by the tubA gene must also be involved in both processes. When benA33 is shifted from a permissive to restrictive temperature, nuclei are blocked in mitosis. Light microscopy of blocked nuclei reveals that benA33 inhibits movement of chromosomes to the poles, and electron microscopy of blocked nuclei shows that they contain apparently normal spindles. Thus benA33 does not block microtubule assembly but, directly or indirectly, blocks microtubule disassembly. BenA33 also confers resistance to several antimicrotubule agents and the heat sensitivity conferred by benA33 is suppressed by each of these agents. We suggest that the simplest explanation for these effects is that benA33 causes a temperature-dependent hyperstabilization of microtubules that blocks chromosomal movement by blocking microtubule disassembly.