A beta-tubulin gene previously suggested to participate in conidial development in the filamentous fungus Aspergillus nidulans is shown to be developmentally regulated in its expression. A quantitative S1 assay was used to show that the abundance of the tubC messenger RNA increases during conidial development relative to the benA messenger RNA. Morphological analysis of cultures, used to prepare RNA for the S1 analysis, demonstrated that the increase in tubC messenger RNA was directly correlated with the appearance of conidiating cell types and structures and that as these cell types become more prevalent in the culture, the level of the messenger RNA increases. Two-dimensional gel electrophoresis of labeled proteins showed that the relative increase in tubC messenger RNA was reflected in an increase in the tubC protein beta 3. RNA blot hybridization analysis and in vitro translation of total RNA from mycelia and conidia were used to demonstrate that both benA and tubC beta-tubulin messenger RNAs are absent from conidia.