Doubletime (DBT), a homolog of casein kinase I[.epsilon] (CKI[.epsilon]), is an essential circadian clock component and developmental regulator in Drosophila melanogaster. The authors cloned a dbt homolog from the silkworm, Bombyx mori( Bmdbt), and examined its spatial and temporal expression in comparison to a CKI[.alpha] homolog ( BmCKI[.alpha]). Four Bmdbt splice variants and 2 BmCKI[.alpha] splice variants were detected, and their expression patterns varied in different tissues. The level of Bmdbt transcript in the brain was constant under LD 12:12 while those of BmCKI[.alpha] transcripts fluctuated with a decrease at ZT12. In situ hybridization showed presumably identical distribution of dbt, CKI[.alpha], and per transcripts in the putative clock neurons of the head ganglia, as well as in the retina, where CKI-and PER-like immunoreactivities were colocalized, suggesting a possible involvement of both CKIs in the B. mori circadian system. Signals were detected at 4 Ia1 neurons in each dorsolateral protocerebrum, 6 to 8 cells in the pars intercerebralis, about 6 cells in the suboesophageal ganglion, 2 neurons in the frontal ganglion, and most of the photoreceptors. All these cells contained dbt, CKI[.alpha], and per antisense transcripts. The Northern analysis of dbtand CKI[.alpha] transcripts at different developmental stages showed that both genes were expressed at relatively high levels during early embryogenesis and in the ovary. The levels of CKI[.alpha] transcripts were also high in the late larval stages until the mid-fifth instar and then suddenly disappeared before larval-pupal ecdysis. In contrast, the transcriptional activity of both genes was low in diapausing eggs.