We have characterized the promoter specificity of the Arabidopsis thaliana alpha 1-tubulin (alpha 1-tub) gene by studying expression patterns of gene fusions between the 2.2 kbp 5' upstream region of the alpha 1-tub gene and each of three different reporters: chloramphenical acetyltransferase, beta-glucuronidase or the diphtheria toxin chain A gene. Analysis of transgenic tobacco and Arabidopsis plants carrying the transgene showed that the chloramphenicol acetyltransferase and beta-glucuronidase activities were not detected in any vegetative or reproductive organs except mature pollen. Transgenic tobacco plants carrying the diphtheria toxin chain A gene under the control of the alpha 1-tub promoter were of normal phenotype but seed fertility was drastically reduced. Furthermore, the transgene could not be transmitted to the next generation through pollen, supporting the observation that the alpha 1-tub promoter is active only in pollen. It was observed that the promoter activity was most active in mature pollen and decreased significantly during in vitro pollen germination, indicating that the promoter is inactive or subdued in germinating pollen. The promoter activity was not affected by various plant growth hormones during pollen maturation.