Crowding refers to the increased difficulty in identifying a letter flanked by other letters. The purpose of this study was to determine if the peak sensitivity of the human visual system shifts to a different spatial frequency when identifying crowded letters, compared with single letters. We measured contrast thresholds for identifying the middle target letters in trigrams, for a range of spatial frequencies, letter separations and letter sizes, at the fovea and 5 degrees eccentricity. Plots of contrast sensitivity vs. letter frequency exhibit spatial tuning, for all letter sizes and letter separations tested. The peak tuning frequency grows as the 0.6-0.7 power of the letter size, independent of letter separation. At the smallest letter separation, peak tuning frequency occurs at a frequency that is 0.17 octaves higher for flanked than for unflanked letters at the fovea, and 0.19 octaves at 5 degrees eccentricity. This finding suggests that the human visual system shifts its sensitivity toward a higher spatial-frequency channel when identifying letters in the presence of nearby letters. However, the size of the shift is insufficient to account for the large effect of crowding in the periphery.