The larval salivary gland secretion of Drosophila melanogaster is separated, by acid-urea polyacrylamide electrophoresis, into several different protein components. The genes specifying three of these had hitherto been located both genetically and cytogenetically, and shown to map close to sites that form prominent puffs in larval salivary gland chromosomes. A fourth component of the secretion (SGS-6), present in some, but not all, stocks of D. melanogaster, is shown to be under the control of a gene that maps to 3-42.0 and to bands 71 C1,2-71 F3,5. In those stocks that possess SGS-6 a puff is active in 71 C3-4 in Puff Stage 1, but not in older larvae. We conclude that this puff is the manifestation of a transcriptionally active Sgs-6 gene.