Humans are able to attentively discriminate number from 6 months of age. However, the age of the emergence of this ability at the pre-attentive stage of processing remains unclear. Event-related potentials (ERPs) were recorded in newborn human infants aged from 3 to 5 days. At 500-ms intervals, the infants were passively exposed to 200-ms sequences of four tones. Each tone could be either 1000 or 1500 Hz in frequency. In most sequences (standards), the ratio of the tones of one frequency to those of the other frequency in a sequence was 2 : 2. In the remaining sequences (deviants, P = 0.1), this ratio was either 3 : 1 or 4 : 0. The mismatch response of ERPs could not be found for 3 : 1 deviants, but it was a robust finding for 4 : 0 deviants, showing the neurophysiological ability of the infants to register the larger deviant–standard difference. The findings suggest very early sensitivity to auditory numerical information in infancy.