ERG's to spectral lights were recorded from all eyes of intact wolf spiders. Secondary eyes have maximum relative sensitivities at 505-510 nm which are unchanged by chromatic adaptations. Principal eyes have ultraviolet sensitivities which are 10 to 100 times greater at 380 nm than at 505 nm. However, two animals' eyes initially had greater blue-green sensitivities, then in 7 to 10 wk dropped 4 to 6 log units in absolute sensitivity in the visible, less in the ultraviolet. Chromatic adaptations of both types of principal eyes hardly changed relative spectral sensitivities. Small decreases in relative sensitivity in the visible with orange adaptations were possibly retinomotor in origin. Second peaks in ERG waveforms were elicited from ultraviolet-adapted principal eyes by wavelengths 400 nm and longer, and from blue-, yellow-, and orangeadapted secondary eyes by wavelengths 580 nm and longer. The second peaks in waveforms were most likely responses of unilluminated eyes to scattered light. It is concluded that both principal and secondary eyes contain cells with a visual pigment absorbing maximally at 505-510 nm. The variable absolute and ultraviolet sensitivities of principal eyes may be due to a second pigment in the same cells or to an ultraviolet-absorbing accessory pigment which excites the 505 nm absorbing visual pigment by radiationless energy transfer.It is now well-known that m a n y insects can see color, but it remains to be proven that another group of arthropods, the spiders, also see color. Peckham and Peckham (1887) thought spiders could see color, since wolf spiders came to rest preferentially under red glass as opposed to blue, yellow, or green glasses. Frings (1941) was unable to repeat the Peckhams' results; he used equally illuminated as well as equally transmitting red, yellow, or blue cellophanes. Schlegtendal (1934) used moving colored stripes to elicit optomotor responses in a variety of arthropods. She was unable to elicit a consistent behavioral response from wolf spiders, much less a color preference, and concluded only that the moving stripes released an escape reflex.The most thorough work on spider color responses is that of K/istner (1950). He used a behavioral approach based on the observation that jumping spiders (Evarcha falcata) would preferentially leap upon and ascend a