Polypyrimidine tract binding protein (PTB) is a member of the hnRNP family of RNA binding proteins (Nucleic Acids Res., 20 (1992) 3671) that functions in a number of processes important for the regulation of mRNA metabolism and gene expression (reviewed in Curr. Biol., 7 (1997) R705). Specifically, PTB binds polypyrimidine-rich intronic elements upstream of alternatively spliced exons to antagonize the binding of the essential U2AF splicing factor and repress the use of the regulated exons in specific tissues (RNA, 1 (1995) 234). Additionally, PTB interacts with elements that mediate 3-prime end processing of nascent transcripts (Mol. Cell. Biol., 19 (1999) 78) and is required for the expression of viral mRNAs that contain an internal ribosome binding site (RNA, 5 (1999) 344; RNA, 1 (1995) 924). Tissue-specific or alternatively spliced isoforms of PTB are thought to have different gene regulatory properties (Proc. Natl Acad. Sci. USA, 97 (2000) 6350; RNA, 7 (2001) 819), but little is known about the function and activity of PTB isoforms during development. Here, we investigate the expression of PTB during Drosophila embryogenesis using in situ hybridization assays. We show that PTB expression is patterned in the early embryo and occurs in specific mesodermal and neuronal lineages as well as in the imaginal discs and adult germline. These data indicate that PTB regulates gene expression in specific tissue lineages during development.