The dinoflagellate luciferase of Lingulodinium polyedrum has three catalytic domains in its single polypeptide chain (M_r = 137 kDa), and each 42 kDa domain is enzymatically active. Deletion mutants for N- or C-terminal regions of domain 3 of the luciferase, ranging from 29 to 38 kDa, were constructed and expressed in E. colicells. The activities of N-terminal deleted mutants were above 20% of wild type, but showed different pH-activity profiles. By contrast, the activities of C-terminal deleted mutants decreased drastically to below 1% of wild type, although their pH-activity profiles and spectra were identical to those of wild type L. polyedrum luciferase. These results indicate that the C-terminal region of this enzyme could be important for the bioluminescence reaction, although based on crystal structure of the luciferase domain, this region does not contain active or regulatory sites.