The gene encoding the silk protein P25 is specifically transcribed in the posterior silkgland of Bombyx during larval intermoults and is repressed during moults. By performing in vitro DNA-protein interactions, at least five putative regulatory elements were localized in the 5' flanking region of the gene. The most proximal element, close to the TATA box, interacts with SGFB, a silkgland-specific factor which could be involved in the tissue-specific expression of the gene. A more upstream sequence is recognized by an ubiquitous factor, BMFA, which exhibits cyclical modifications in relation to the moulting cycle and could thus be involved in the temporal control of the gene during the development. A construct containing a reporter gene fused to 1450 bp of P25 5' flanking sequences was integrated into the Drosophila genome and shown to be specifically expressed in the larval salivary gland, the organ homologous to the silkgland. Recurrent deletions of this construct showed that the proximal 254 bp contain all the sequences required for this specific expression. Similar foreign constructs introduced in the silkgland in vivo by a particle delivery system were specifically transcribed in the posterior silkgland but remained silent in the middle silkgland as the endogenous genes. This methodology will be used to assay the function of the defined cis-acting elements in the spatial regulation of expression of P25.