Cells from three different layers of the retina were tested for regular horizontal arrangement or random distribution. Monkey cones, cat cones, cat A-type horizontal cells and cat alpha-ganglion cells are all arranged in a regular mosaic. This was assessed by measuring the distance to the nearest neighbour of each cell. The nearest neighbour distributions differed significantly from those of random dot patterns. The precision of the mosaic decreased in the order monkey cones, cat cones, cat A-type horizontal cells, cat alpha-ganglion cells. All four cell types were - independent of density - more regularly arranged in the centre of the retina than in the periphery. The advantages of a regular arrangement compared with a random distribution and mechanisms which could generate a regular mosaic are discussed.