To determine the developmental level at which letter processing skills become automatic, an experiment was conducted using a variant of the visual search task. Subjects in grades one, two, and three and in college searched for target letters displayed on a cathode ray tube along with either visually confusable letters, acoustically confusable letters, or digits. Accuracy and reaction time of search were evaluated for evidence of the visual search "category effect," or the facilitation when the target differed from the distractor items with which it was presented. The findings revealed that (1) the visual search task was a sensitive and reliable measure of the ability of young children and adults to discriminate visually presented stimuli on the basis of visual features and categorical dimensions; (2) when category distinctions had to be maintained in memory or were not presented simultaneously in the display, subjects of all ages had difficulty using the differences in searching; and (3) subjects without a great deal of experience with stimuli such as letters and digits showed the "category effect" in the same way as older, more experienced subjects. (FL)