The dark-reaction (pigment aggregation) of isolated, light-sensitive pigment cells of the sea urchin Centrostephanus longispinus was monitored using IR-microscopy. It differs in time course and mode of pigment displacement from the inverse process of pigment dispersion in white light. Partial illumination of darkness respectively causes locally different and appropriate pigment movements in the individual cell regions. Spot-illumination of a cell process of an otherwise darkened chromatophore leads to local pigment dispersion (from thd dark-adapted state) or aggregation (from the light-adapted state) exactly to the point of stimulation, while the pigment cell shows normal dark-reaction in all other regions. The results indicate that the pigment granules do not bear the receptor pigment, rather it may be expected in the whole cell membrane. The observation of contrary translocations in different regions of the cell support the concept of a local mechanism of granule movement regulated in a very precise manner.