Two chitin synthase (CHS) genes of the red flour beetle, Tribolium castaneum, were sequenced and their transcription patterns during development examined. By screening a BAC library of genomic DNA from T. castaneum (Tc) with a DNA probe encoding the catalytic domain of a putative Tribolium CHS, several clones that contained CHS genes were identified. Two distinct PCR products were amplified from these BAC clones and confirmed to be highly similar to CHS genes from other insects, nematodes and fungi. The DNA sequences of these genes, TcCHS1 and TcCHS2, were determined by amplification of overlapping PCR fragments from two of the BAC DNAs and mapped to different linkage groups. Each ORF was identified and full-length cDNAs were also amplified, cloned and sequenced. TcCHS1 and TcCHS2 encode transmembrane proteins of 1558 and 1464 amino acids, respectively. The TcCHS1 gene was found to use alternate exons, each encoding 59 amino acids, a feature not found in the TcCHS2 gene. During development, Tribolium expressed TcCHS1 predominantly in the embryonic and pupal stages, whereas TcCHS2 was prevalent in the late larval and adult stages. The alternate exon 8a of TcCHS1 was utilized over a much broader range of development than exon 8b. We propose that the two isoforms of the TcCHS1 enzyme are used predominantly for the formation of chitin in embryonic and pupal cuticles, whereas TcCHS2 is utilized primarily for the synthesis of peritrophic membrane-associated chitin in the midgut.