The visual response properties of cells in the cat's dorsocaudal claustrum were studied physiologically. Quantitative observations were made of 55 cells, and qualitative observations were made on 228 others. The claustral cells formed a physiologically homogeneous population. The overwhelming majority were orientation selective, and most also showed a striking preference for long stimuli, their responses summating up to lengths of 40 degrees or more. Moving stimuli were always much more effective than stationary ones. In other respects, claustral cells were tolerant of wide variation in stimulus features. Their responses were about equally brisk to either direction of movement of a properly oriented stimulus, and the velocity of movement was likewise not critical. They appeared not to summate across the dimension of their receptive fields orthogonal to the preferred orientation so that narrow or broad slits, or edges, evoked similar responses. Dark slits on light backgrounds were as effective as light slits on dark backgrounds. Finally, a large majority of cells were driven equally well by either eye. These properties of claustral cells differ in several respects from those of their principal targets, cells in layer IV of visual cortex.