Embryonic and larval expression patterns of the sericin-1 gene and its presumed transcription factor, SGF-3/POU-M1, in the silk gland were analyzed by in situ hybridization and immunohistochemistry. The sericin-1 transcripts were first detected at embryonic stage 26 in an increasing gradient pattern in the middle and posterior part of the middle silk gland (MSG), while at the same stage the SGF-3/POU-M1 was already present in the entire anterior silk gland (ASG) and in the MSG but with a decreasing gradient pattern. The latter expression pattern was consistently maintained through all larval stages, while the sericin-1 expression was detected during the feeding stages but disappeared at the molting stages. These observations suggest that, although the SGF-3/POU-M1 was proposed to be a positive transcription factor for the sericin-1 gene, the protein might function in a negative manner on sericin-1 gene transcription. Alternatively, it is also possible that the sericin-1 gene might require another unidentified factor or mediator for in vivo transcription.