To understand the development of binocularity we must understand the relationship between the development in early infancy of its various components (basic ocular alignment, disparity-induced convergence, binocular fusion, and stereopsis). Accordingly, we have compared basic eye alignment and convergence in a group of 34 healthy infants and the age of onset of convergence and binocular fusion preference in a group of 89 infants. In the first experiment the Hirschberg test showed that most infants are orthotropic from our youngest testing age of 2 weeks while full convergence is not seen until 6 to 16 weeks. In experiment two, mature sensory binocularity has a sudden onset for individual infants at 12.7 +/- 2.5 weeks of age. Smooth complete convergence indicative of disparity control usually occurs shortly after the onset of sensory fusion (13.7 +/- 2.5 weeks) with a correlation between the age of onset of these two functions of r = 0.59. Both onsets occur earlier for females than males. Our data suggest that infant eyes are capable of alignment very early in life. However, mature sensory binocularity and accurate convergence first appear together at about 3 months of age indicating that they share a common causal mechanism which probably involves refinements in striate cortex circuitry.