Functional magnetic resonance imaging (fMRI) was used to examine the pattern of activity of prefrontal cortex in prepubertal children during performance of a nonspatial working memory task. The children observed sequences of letters and responded whenever a letter repeated with exactly one nonidentical letter intervening. In a comparison task, subjects monitored similar sequences of letters for any occurrence of a single, prespecified target letter. Location of activation closely approximated that observed in a recent fMRI study with adults using exactly the same task. Activation of the inferior and middle frontal gyri was reliably observed within individual subjects during performance of the working memory task relative to the comparison task. Activation increased and decreased with a time course that was highly consistent with the task manipulations and correlated with behavioral performance. To our knowledge, this study is one of the first to demonstrate the applicability of fMRI to a normative developmental population. Issues of age dependence of the hemodynamic responses of fMRI are discussed.