Abstract Three different karyotypes have been found so far among Saimiri originating from five different South American localities. All animals examined have the same diploid number (44) of chromosomes but the number of acrocentric and submetacentric chromosomes varies, presumably as a result of pericentric inversions. Saimiri originating from Iquitos, Peru, consistently have ten acrocentric chromosomes; animals originating from Leticia, Colombia, have 12 acrocentric chromosomes. Hybrids produced in our laboratory have the expected 11 acrocentrics and one unpaired submetacentric chromosome. Animals originating from Guyana have fourteen acrocentric chromosomes and the expected two fewer submetacentric chromosomes. Squirrel monkeys from Costa Rica, Panama, and Pucallpa, Peru, studied to this date conform to the Iquitos type with ten acrocentric chromosomes. These findings point to genetic differences which may result in variable responses to laboratory situations. The evolutionary factors involved in this rearrangement of chromosomes and possible influences on phenotypes are subjects of interest for future study. The importance of identifying the source of squirrel monkeys used in biomedical research is apparent if results from different laboratories are to be repeated or compared.