OBJECTIVE To measure Vernier acuity and resolution development after 3 years of age. METHODS Observers were 39 children with normal vision (aged 3 to 12 years), 10 adult observers with normal vision (aged 19 to 24 years), and 7 adults with amblyopia. Vernier acuity and resolution were measured using uncrowded static stimuli and a 3AFC psychophysical paradigm. Curve fitting was used to estimate A2, the age at which thresholds are twice asymptotic levels. RESULTS Vernier acuity was hyperacute (i.e., finger than predicted from foveal cone size or spacing) in 3- to 4-year-old observers, but developed later (A2 = 5.6 +/- 1.5 years) than resolution acuity (A2 = 2.2 +/- 0.9 years). CONCLUSIONS Children's Vernier thresholds are poorer than would be predicted solely from their decreased foveal photon capture. Therefore cortical immaturity may play a role in children's relative position acuity deficit. R/V ratios (resolution/Vernier thresholds) for the youngest age group are similar to those for adult nonstrabismic amblyopes, but better than for strabismic amblyopes.