20E) plays a role in the direct regulation of crustacean molting and growth. The synthesis and degradation of 20E is catalyzed by cytochrome P450 enzymes (CYP enzymes). We attempted to determine the molecular mechanism for the growth promoting effect of KK-42 in) (body length 3.55.0 cm) were exposed to a solution containing KK-42 (1.95×10 ^{mol/L) or to a control solution without KK-42 then cultured normally. A single was cloned and its spatiotemporal expression was analyzed using real-time PCR. The hemolymph 20E titer was measured by reverse-phase high performance liquid chromatography. The full-length cDNA contained a 1 545 bp open reading frame encoding a predicted protein of 515 amino acids which shares common characteristics with the cytochrome P450 superfamily. The expression of mRNA was highest in the hepatopancreas. Following KK-42 treatment, expression of hepatopancreas decreased significantly to >72.3 % that of the control group and was that of the control on day 6. Administration of KK-42 resulted in an increase in hemolymph 20E titers compared to that of control animals. Our results suggest that the inhibition of hepatopancreas expression and the increase in hemolymph 20E titer following KK-42 administration may explain the growth promoting effect of KK-42 in}