MEASURED BY A MODIFIED METHOD OF LIMITS, MINIMUM SEPARABLE VISUAL ACUITY OF 12 MACACA MULATTA (.65' OF ARC) WAS SLIGHTLY SUPERIOR TO THAT OF 12 SAIMIRI (.74' OF ARC). RANKED ACUITY SCORES WERE SIGNIFICANTLY DIFFERENT (P < .01). SIMILARITY OF ACUITY IN THE 2 GROUPS CORRELATES WELL WITH THE IDENTICAL CORTICAL MAGNIFICATION FACTOR FOR THE FOVEA IN THE 2 SPECIES, I.E., LINEAR AMOUNT OF VISUAL STRIATE CORTEX TO WHICH FOVEA PROJECTS; THE DIAMETER OF THE EYE IS DIFFERENT IN THE 2 SPECIES (APPROXIMATELY 20 MM. IN MACACA VS. 14.5 MM. IN SAIMIRI), INDICATING THAT THE SQUIRREL MONKEY IS RESOLVING A FINER RETINAL IMAGE. (PsycINFO Database Record (c) 2006 APA, all rights reserved)