The melanophores of a freshwater teleost, Zacco temmincki, were photosenstitive: In darkness the melanophores induced a melanosome aggregation and in light they caused a melanosome dispersion. Processes surgically severed from the cells also responded to changes in illumination in the same manner as the cell bodies or the intact cells did. This result indicated that photoreceptive sites were distributed over the whole of the cell, and that the nucleus did not directly take part in the light response, in both dark-induced aggregation and light-induced dispersion.