1. The spectral sensitivity of the chromatophores of Diadema setosum (Leske) was studied, using monochromatic light.

2. The chromatophores are most sensitive in the region of the visible spectrum between 450 and 500 mµ with a maximum at 470 mµ.

A part of this work was supported by a grant-in-aid for Fundamental Scientific Research of the Ministry of Education.