Spectral sensitivity was measured under different adaptation levels using a behavioral training technique in which the fish had to discriminate between a dark test field and a test field illuminated with monochromatic light. Depending on which of the two test fields was used as training test field, two functions were obtained which differ (1) in absolute sensitivity and (2) in shape. When trained on the dark test field, the fish seems to discriminate on the basis of a "color" cue, but it uses a "brightness" cue when trained on the illuminated test field. This was concluded from measurements of wavelength discrimination. Under low levels of the adaptation light (1.5 and 0.2 lx instead of 20 lx), the L-cone type contributes to perception of "brightness" but not to color vision. This difference in the adaptation behavior in the long-wavelength range was used to identify the ganglion cells which may represent channels for "brightness" and "color" in the retina. Action spectra were recorded extracellularly at different levels of dark- and light-adaptation.