1. Adaptive changes in shade occur in Fundulus heteroclitus when placed on white, gray, and black backgrounds. This adaptation is brought about by the contraction or expansion of the melanophores. 2. On colored backgrounds for short periods, adaptation in only shade occurs. 3. Prolonged stimulation by colored backgrounds brings about adaptation in color. This occurred in yellow, red, green, and blue environments. 4. The experiments with spectral lights of different wave-lengths but of the same radiant energy or intensity, show that the melanophores respond similarly at all wave-lengths, the contraction being due to light intensity. 5. The response to the longer wave-lengths, red and especially yellow, in causing an expansion of the xanthophores, though intensity was the same as in the blue on short wave-length end of the spectrum, shows that the quality of light or wave-length has a specific influence on the coloration of Fundulus. 6. The stimuli causing changes in shade and color are received through the eyes. 7. The changes in color are brought about by the degree of expansion or contraction of the melanophores and xanthophores combined with the optical properties of the guanophores or iridocytes and the reflecting layer. The guanophores are frequently found associated with the melanophores. 8. Adaptive change to a yellow background is brought about by a maximum expansion of xanthophores and a maximum contraction of the melanophores. 9. In fishes adapted to a red background, the xanthophores are only partially expanded and the melanophores contracted. The pink color is due chiefly to a dilation of the blood capillaries. 10. In fishes adapted to a green background, the green tints present in the normal light adapted fish, is increased and is especially noticeable above the eyes. The xanthophores and melanophores are partially expanded. 11. In blue adapted fishes the xanthophores are maximally contracted and the melanophores in the deeper portions of the dermis and upon the proximal parts of the scales, are expanded. Postscript. Since the preparation of this paper I have had the opportunity of repeating the experiments on Fundulus at the St. Andrew's Biological Station. My results have been the same as those obtained in the Harvard Laboratory-except that I failed to get on a red background the pink coloration and vaso-dilation described in this paper. Whether this is a seasonal or a local difference in the fishes remains to be worked out.