This study investigated the size of the visual region within which adults use visual information to distinguish among letters as they read. Fifteen college students read passages from a cathode-ray tube as their eye movements were monitored. On occasional fixations, letters in specified regions were replaced by other letters. The effects of this manipulation were observed on their eye movement patterns. Erroneous letters lying four or more letter positions to the left of the fixated letter, or eight or more to the right, had no discernible effect on reading. There was no evidence to indicate that prior research in which letters were replaced on every fixation had underestimated the size of the perceptual span. The fact that the present study yielded a smaller estimate of the perceptual span and showed smaller effects from letter replacement than have previous studies is explained by methodological differences in the aspect of the text stimulus being studied.