The onset of measurable stereopsis in human infants occurs at approximately 4 months of age, directly following a period of rapid oculomotor development. The experiment reported here was designed to investigate whether the onset of stereopsis is determined solely by the onset of accurate oculomotor coordination or whether neural development in the binocular visual system is necessary. These alternatives were examined by means of a testing procedure which ensured that stimuli were presented within 1.4 deg of the horopter. That these stimuli are insensitive to errors of vergence was verified by testing thirty infants aged 6 to 10 months who were known to have stereoacuity of at least 1 min of visual angle. All but one infant retained the ability to make stereo discriminations with simulated vergence errors of up to 30 prism diopters. Results obtained from a group of forty-four infants tested longitudinally between 0 and 6 months showed a mean age of onset of stereopsis of 4.1 months. Thus, ensuring that stimuli are presented near to the horopter does not significantly alter estimates of the age of onset of stereopsis. These results suggest that neural development which is critical for the ability to make stereo discrimination must occur during the first 3 months of life.