1. Recordings were made from single axons of 218 ganglion cells in the optic nerve of the turtle, Pseudemys scripta elegans. Responses to light stimuli were used to classify 182 of the cells into four functional groups.2. Movement-sensitive cells (113 units) responded best to moving stimuli independent of the stimulus colour or direction of motion. Their receptive fields were organized into antagonistic centres and surrounds with the same spectral sensitivity. Based on their patterns of response they were of three types: ON-centre, OFF-centre, and ON-OFF.3. Directionally selective cells (sixty-five units) responded best to stimuli that moved in one preferred direction and not to stimuli that moved in the opposite direction. Their receptive fields had antagonistic centres and surrounds with the same spectral sensitivity. An area of silent inhibition could be shown on one side of the receptive fields. The preferred directions appeared to fall into three groups separated by about 120 degrees. Response patterns were of two types; OFF-centre and ON-OFF.4. Sensitizable colour cells (two units) had centre-surround organization with red-green opponent responses from both the centre and the surround (double opponent cells). In addition, the sensitivity to either of the opponent colours was effectively turned on or off by a steady background of the other opponent colour.5. Orientation cells (two units) responded best to moving bars of specific orientations in the visual field. Their receptive fields consisted of adjacent mutually antagonistic areas with linear boundaries.6. Under photopic conditions all of the ganglion cells were most sensitive to light near 630 nm wave-length, the spectral maximum of the red-sensitive single cones in the turtle retina. About 75% of the cells had a secondary spectral peak near 560 nm, the spectral maximum of the green-sensitive single cones. Three cells had unusually broad sensitivity extending into the blue portion of the spectrum. Over half of the cells received additional input from rods as evidenced by a shift in spectral sensitivity and increases in latencies and receptive field sizes with dark-adaptation.7. Measurements of the absolute sensitivity of the ganglion cells showed that a threshold response required absorption of several hundred photons in the red-sensitive cones of the receptive field centre. After 10 min dark adaptation the rods required absorption of about 20 photons.