From 67 diploid male Chinese hamster cells in vitro, an idiogram was constructed. The chromosomes can be classified into 4 natural groups: group I, 2 pairs of large submetacentrics (1 and 2); group II, 3 pairs of medium-sized submetacentrics (the sex pair, 4 and 5); group III, 3 pairs of medium-sized subtelocentrics (6,7, and 8); group IV, 3 pairs of small metacentrics (9,10, and 11). A system of subdividing each chromosome into regions is also suggested.