Visual pigments were extracted from the retinae of a large and diverse sample (179 species, in 40 families) of tropical marinefishes. By the method of partial bleaching, most of these were found to be retinal-1 pigments, with their maxima tightly grouped between 489 and 502 nm; but a few have λ max at shorter wavelengths. The spectral position of these pigments does not correlate with diurnal or nocturnal activity patterns, nor do closely related species necessarily have similar pigments. In general, the deeper-living species have rhodopsins with λ max at shorter wavelengths, a correlation with the bluer quality of light in deeper water. The spectral distribution of light was measured in air and underwater at a coral atoll. During twilight the underwater spectrum narrows and shifts to shorter wavelengths, as a result of increased absorption of yellow-orange light by the atmosphere. This effect at twilight is unique among the general spectral events that constitute each day. The behavioral patterns of reef fishes make clear the relationships between underwater light and the visual pigments. During evening twilight, there is an orderly sequence, in which the diurnal fishes seek cover, and then, after a “Quiet Period”, the nocturnal species emerge. The “Quiet Period” is the most critical and visually difficult time because light intensities are transitional between the thresholds of photopic and scotopic vision. Predation reaches a maximum during the “Quiet Period”. The rhodopsins of all these fishes tend to match the background light available during evening and morning twilight. Photochemically, visual sensitivity is maximized and, as a result, predation is held in check. The relationships between light, behavior and visual physiology are discussed. Although their rhodopsins are spectrally similar to those of the prey, crepuscular predators obtain a visual advantage by having moderate numbers of very large cones, which provide adequate sensitivity and sufficient resolution for motion detection.