We obtained 305 expressed sequence tags (ESTs), which are from the poly(A) site to the most proximal MboI site, from mycelia at the early sexual developmental (ESD) stage of Aspergillus nidulans. By comparison of these ESTs with those obtained previously from the vegetative stage and from the late sexual developmental stage followed by Northern blot analyses, genes of 17 ESTs were identified as being expressed more abundantly at the ESD stage than at the vegetative stage. Five of 17 genes were expressed more abundantly in the presence of the veA gene or the nsdD gene, suggesting that these 5 genes may be involved in sexual development. In a gene of one EST, appearing three times among 305 ESTs and identified by GenBank, polyadenylation seemed to occur at two sites. Nucleotide sequences of the gene having the EST and its cDNA revealed that the gene can code for a 202-amino-acid polypeptide with an estimated molecular mass of 23 kDa. The deduced amino acid showed 73% identity to Saccharomyces cerevisiae ribosomal protein L16A (RPL16A), and therefore the gene was named rpl16a. A. nidulans RPL16A had a putative leucine zipper motif and a basic leucine zipper motif like those of other organisms. The expression level of the rpl16a gene, present as a single copy in this organism, reached a maximum after 2 h, decreased thereafter, and increased again 30 to 50 h after the end of induction of sexual development. These results clearly indicated that the rpl16a gene is expressed differentially during sexual development.