We have mapped 17 extragenic suppressors of benA33, a heat-sensitive beta-tubulin mutation of Aspergillus nidulans, to the tubA alpha tubulin locus. Fifteen of these tubA mutations cause cold sensitivity in a genetic background with benA33 and appear to cause lethality in a background with the wild-type benA allele. We examined the microtubule-mediated processes, nuclear division and nuclear migration, in seven different cold-sensitive double mutants, each carrying benA33 and a different cold-sensitive tubA allele. Nuclear division and migration were inhibited at a restrictive temperature in each case, suggesting that cold sensitivity is due to the inhibition of microtubule function at low temperatures. A single allele, tubA4, suppressed the heat sensitivity conferred by benA33 but did not confer cold sensitivity in a benA33 background, however in a wild-type benA background, tubA4 conferred supersensitivity to antimicrotubule agents and weak cold sensitivity. TubA4 did not suppress the heat sensitivity conferred by two other benA alleles. The cold sensitivity conferred by tubA4 was suppressed by the microtubule stabilizing agent deuterium oxide, and the suppression of heat sensitivity conferred by four other tubA mutations was reversed by deuterium oxide. These results suggest that these mutations may affect hydrophobic interactions between alpha- and beta-tubulin.