To elucidate the roles of DEC1 and DEC2, basic helix-loop-helix transcription factors, in the circadian clock of photosensitive zebrafish peripheral cells, zebrafish Dec1 and Dec2 (zDec1 and zDec2) were cloned and their functions and expression patterns were examined in BRF41, a zebrafish cell line. zDEC1 and zDEC2 have high sequence similarity to mammalian counterparts and the molecular phylogenetic analysis of the zDEC1 and zDEC2 sequences reflected the predicted pattern of species classification. zDEC1 and zDEC2 inhibited zCLOCK1:zBMAL3 mediated transcription as CRY1a. zDec1 and zDec2 mRNA showed robust circadian oscillation in BRF41 cells. However, zDec1 and zDec2 mRNA was not strongly induced by exposure to light. These results indicate that zDec1 and zDec2 are involved in the cicadian clock mechanism in photosensitive zebrafish peripheral cells by suppressing CLOCK/BMAL-induced gene expression and that the feedback loops of zDEC1 and zDEC2 may be interlocked with the PER/CRY core circadian feedback loops. (c) 2006 Elsevier Inc. All rights reserved.