The present investigation was undertaken to determine whether significant differences in auditory processing and perceptual abilities exist between (1) stutterers as a supposedly homogeneous group when compared with controls, (2) two differentiated subgroups of stutterers, and (3) either of the stuttering subgroups when separately compared with controls. Dichotic listening and masking level difference (MLD) tasks were administered to the two groups of school-age stutterers and an age-matched nonstuttering control group. Stuttering subjects were differentiated into "organic" and "functional" subgroups on the basis of neuropsychological test performances. Organic stutterers performed significantly poorer than did controls on one MLD experimental condition. Functional stutterers performed more like control subjects than like organic stutterers. Starkweather, Hirschman, and Tannenbaum, 1976), atypical performance on neuropsychological tests (Daly and Smith, 1976; Daly, Kimbarrow, and Smith, 1977), lack of cerebral dominance (Curry and Gregory, 1969; Brady and Berson, 1975), and dysfunction of auditory processing and perceptual abilities (Hall and Jerger, 1978; Toscher and Rupp, 1978). The hypothesis that some type of organic dysfunction may