Four patients in three different families had a form of autosomal recessive incomplete achromatopsia not previously described. The visual acuity was 6/18 to 6/60 (20/60 to 20/200) with minimal ophthalmoscopic abnormality and normal fluorescein angiogram. The photopic electroretinographic responses were present in all four patients; the fusion rate of 60 Hz was only slightly subnormal. The high-intensity scotopic response was subnormal. The patients failed color screening plates and accumulated over 400 errors with scotopic axis on the Farnsworth-Munsell 100-hue test. The Rayleigh match was abnormal, displaced toward the red primary, but with normal luminance. The photopic luminous efficiency function was similar to that of the deuteranope. Color matching revealed a trichromatic form of color vision mediated by long wavelength and short wavelength cones, and a rhodopsin receptor.