During Drosophila spermatogenesis transcriptional activity is mainly restricted to premeiotic stages. Translation during sperm morphogenesis, however, proceeds for several days, requiring a high stability for mRNAs translated postmeiotically. We studied expression of the Drosophila beta 2 tubulin gene, which is expressed solely in the male germ line from the primary spermatocyte stage onwards. Cis-acting elements involved in the regulation of mRNA levels were investigated in transgenic fly strains. In adult testes, mRNA amounts from beta 2-lacZ fusion genes in the presence of an 18-bp AT-rich element, termed beta 2DE1, are elevated about threefold. The element is present at about the same position in the 5' untranslated regions of the beta 2 tubulin genes of the distantly related species Drosophila melanogaster and Drosophila hydei. Changing the position of the element on the mRNA reduces the stabilizing effect, while inversion of the beta 2DE1 abolishes its function. The element also acts in a combination with the beta 1 tubulin transcription start site, and the beta 2UE1, which is required to achieve tissue-specific expression. In all experiments done, the comparison of premeiotic with postmeiotic stages strongly implies that this element is involved in regulating mRNA stability. This mRNA stabilizing element acts in a position-independent manner and also on a heterologous mRNA, showing its autonomous functional activity.