To provide further information concerning the sensitivity of SDR methodology to performance differences of closely related species among the New World monkeys, the spatial SDR performances of Owl monkeys were measured and compared to those of Squirrel and Capuchin monkeys previously reported. The error scores of the Owls were more similar to those of Squirrel monkeys, than to the Capuchins, which provides some support for the view that the Squirrel is taxonomically more closely related to the Owl than to the Capuchin. © 1969, Psychonomic Journals, Inc.. All rights reserved.