A research on the acquisition of morphemes of plural, diminutive, augmentative, gerund, imperfect, and preterite with 109 Spanish children from 3 to 6 is reported. The results, obtained using a technique similar to that of Berko (1958), are compared with existing data in other studies that permit us to analyze several processes. These confirm any of the "operating principles" proposed by Slobin (1973, 1985) and seem to explain certain errors that children commit in the ages studied. Data are viewed in a developmental pattern of reorganization of linguistic knowledge.