The chromosome numbers and karyological characteristics of nine species of monkeys (including two subspecies of one species) all in the family Cercopithecidae have been determined in tissue cultures of kidney epithelial cells. For cytological studies, two types of materials were used: (1) cell suspensions obtained by removing cells from the culture vessel walls by incubation with trypsin or versene solutions and (2) cells proliferating on cover slips in depression (Leighton) tubes. In both procedures cells were pretreated with hypotonic saline to spread chromosomes and smear-stained with acetocarmine. The chromosome numbers of the materials studied appeared to be essentially constant with very occasional polyploid and aneuploid cells. Diploid chromosome counts have revealed a notable variation in chromosome number among the species studied. The diploid number 42 occurs in three genera, Macaca (1 species), Papio (2 species), and Cercocebus (1 species); the number 54 in one genus, Erythrocebus (1 species); whereas the fifth genus studied, Cercopithecus, has species with two numbers, 60 (2 species) and 66 (2 species). Details of comparative chromosome morphology are discussed. The fact that all diploid chromosome numbers in these species are multiples of six suggests polyploidy as a possible evolutionary mechanism, but several lines of evidence against this interpretation are presented.