Multielectrode recordings are required to determine how the retina transmits multiple parallel visual images to the brain in the ensemble activity of multiple distinct classes of retinal ganglion cells (RGCs). However, the promise of multielectrode recording is limited by the small numbers of electrodes (dozens) available in state of the art systems, and the consequent recording of just a few examples of each cell type in a single preparation. Here we present the first simultaneous recordings from hundreds of mammalian retinal ganglion cells using a 512-electrode array system developed for this purpose (Litke et al, SFN 2003), as well as receptive field characterization and cell type classification obtained with these recordings. Extracellular recordings were obtained from isolated guinea pig retinas stimulated with the optically reduced image of a computer display. Action potentials from different RGCs were automatically identified and sorted using custom software, and isolation was verified by the presen...