Contrast sensitivity functions (CSFs) of 25 infants were measured longitudinally at 4, 6 and 8 months of age using a preferential-looking method and the method of constant stimuli. Sine-wave gratings varied from 0.27 to 4.32 c/deg, contained eight unattenuated cycles (with edges tapered to uniform gray), and rose to the desired contrast in 2 sec. (1) The average CSF was described on log-log coordinates by a band-pass function. With development it increased in overall sensitivity to contrast, shifted its peak toward slightly higher spatial frequencies, and increased its high frequency cutoff. (2) Log sensitivity at the CSF peak was slightly higher for female than male infants at 6 months, consistent with the hypothesis that vernier acuity (which also may differ between the sexes at this age) is partly mediated by analyzers tuned to low frequencies. (3) Within age groups the individual differences were such that log sensitivities for neighboring spatial frequencies generally correlated more highly than distant frequencies. With development the correlations among distant frequencies below 1.0 c/deg increased. Monte Carlo simulations of a model that shifts spatial analyzers to higher frequencies with age reproduced these results but simulations of adultlike, unshifting analyzers did not. (4) Measures taken 2 months apart tended to correlate more highly than those taken 4 months apart, though some individual differences in the CSF peak remained stable over 4 months.