Earlier observations suggested there might be significant within-species variations in visual sensitivity among squirrel monkeys (Saimiri sciureus). Reported here are the results from measurements of increment-threshold spectral sensitivity in 41 squirrel monkeys of Peruvian origin. As determined in a forced-choice discrimination task, no large variations were found among these animals in sensitivity to a 540 nm test light. However, there were large individual variations in sensitivity to a 640 nm light, the result being that, across this sample of monkeys, the differences in sensitivity to the 540 and 640 nm lights are bimodally distributed. This variation in sensitivity is gender related in that the more sensitive mode in this distribution is made up exclusively of female monkeys. The less sensitive animals include all of the males tested and a minority (7 out of 22) of the females examined. Additional experiments involving the measurement of sensitivity to monochromatic lights under conditions of dark and chromatic adaptation indicate that (a) the variation in sensitivity is not due to the variable presence of an intraretinal filter, and (b) the more sensitive squirrel monkeys have two differentially adaptable spectral mechanisms in the green to red portion of the spectrum, while the less sensitive monkeys do not.