A colony of Macaca nemestrina monkeys was screened for naturally occurring strabismus. Thirteen cases of naturally occurring strabismus were documented, 12 esotropes and one exotrope. The characteristics of the strabismus in these monkeys were similar to those of human clinical cases. Four of the affected monkeys showed interocular differences in grating acuity. We estimated the incidence of strabismus in the monkey colony to be 4%.