We present a detailed characterization of the structure and expression of gene 3 from the 67B locus of Drosophila melanogaster. Northern blot analysis reveals a major poly(A)+ transcript during two stages of development: mid-embryogenesis and beginning of pupation. After heat shock, the abundance of this mRNA is increased and small amounts of larger RNAs representing alternate terminations of the major transcript appear. In Schneider 3 tissue culture cells, beside the major transcript, we also observe small amounts of the larger RNAs after a heat shock. The sequencing of cDNA and genomic clones shows an intronless transcription unit with one long open reading frame. The deduced polypeptide has 169 amino acids. It shares a strong homology with the four small heat shock proteins in the region also conserved in the mammalian alpha-crystalline B2 chain. In gene 3, this homology is restricted to the first 50 residues along the conserved 83 amino acid stretch. Two heat shock regulatory elements are localized upstream from the gene.