Wavelength discrimination was measured with two individuals using a behavioral training technique. The delta lambda function has three minima, indicating best discrimination at 400, 510 and at 570 nm (S2) or 600 nm (S1). In the range between 450 and 510 nm the ability to discriminate wavelengths was absent. This can be explained by the filter effect of the colored oil droplets, as indicated by a model computation. Color mixture experiments revealed that the high discrimination ability at 400 nm is based on an ultraviolet sensitive photoreceptor which is the fourth cone type involved in turtles' color vision. Spectral sensitivity in the ultraviolet region (measured for one turtle) is maximal at 370-380 nm.