We have identified a class of retinal ganglion cells in the chick retina that can be labelled by NADPH-diaphorase histochemistry. These cells have a remarkable topographic distribution, being restricted to the dorsal hemiretina, and form a highly regular mosaic, as revealed by the analysis of nearest neighbour distribution and Delaunay triangulation. Autocorrelation analysis of the mosaic of NADPH-diaphorase-positive retinal ganglion cells shows that the mosaic spatial organization could be generated with the single constraint that two elements cannot be closer than a given minimal distance (dmin), which we confirmed by computer simulations. In contrast with what has been observed in other mosaics, here dmin varies with cell density. However, the observed variation of the exclusion area is consistent with an original assembly of the mosaic with a constant dmin (as is the case in other mosaics), followed by differential expansion of the retina during development.