To investigate the genetics of male sex determination and stamen development in the dioecious plant Silene latifolia (white campion), maleβspecific transcripts were isolated from developing flowers by cDNA subtraction. One of the cDNAs identified, STA1, had high DNA and amino acid sequence homology to the male sex determining gene of Zea mays (maize), TASSELSEED2. Both genes are expressed in male and not in female flowers, However, they do not share the same expression pattern. The TASSELSEED2 gene product is expressed in the gynoecium primordia of male maize flowers where it is necessary for pistil abortion. STA1 is not expressed in the gynoecium primordia of male white campion and therefore its gene product cannot perform the same function in sex determination that TASSELSEED2 performs in maize. STA1 is expressed in tapetal cells of white campion male flowers and of white campion hermaphroditic mutants. A homologous gene is also expressed in the tapetum of hermaphroditic Silene species. Tapetal expression of a homologous gene (named ATA1) was also found in Arabidopsis thaliana. The similarity in primary sequence and expression pattern of STA1 and ATA1 indicate that these genes have a conserved role in tapetum development.