Knowledge of English morphology was studied in intellectually retarded (IQ 50 to 80), normal (IQ 90 to 110), and superior (IQ 120+) children at four chronological age (CA) levels from 4 to 12. The task involved using inflectional and derivational suffixes at two levels of generality: producing inflected and derived forms of English words, and applying rules of English morphology to provide inflected and derived forms of new (nonsense) words. Results indicated that some suffixes were used with perfect accuracy by younger children, whereas other suffixes were not used accurately even by older children; however, for most suffixes accuracy of use increased with age. Some suffixes were used by all three groups with similar accuracy at earlier CA's; most were used more accurately at the earlier CA's by the more intelligent children with the differences disappearing at later CA's; and some were used more accurately by the more intelligent children at both the earlier and later CA's: Study of the variations in generality revealed that, although some suffixes were used more accurately with English words than with new words, most suffixes were used with similar accuracy at both levels of generality. Accuracy in using suffixes differed among morphemes, with larger differences for younger, less intelligent children and smaller ones for older, more intelligent children. (Author/JD)