Injections of tritiated tracer precursors into one eye revealed a highly variable lamination pattern of the LGN in the marmoset Callithrix. In one specimen, only four cell layers were apparent with some indication of further differentiation. In a second specimen, six almost complete relay layers were found in the occipital portion of the LGN. Three other animals showed stages between these two extremes. The lamination pattern of Callithrix thun represents an intermediate stage between a four-layered LGN suggested as the basic primate pattern, and the advanced six-layered LGN of most other anthropoid monkeys. In addition, bilateral retinal terminations were found consistently in layer 0, and contralateral terminations in a narrow band between the magnocellular and parvocellular portions of the nucleus, and within the interlaminar zone which separates the two magnocellular layers.