A study of the processing of morphophonology by native speakers and second language learners focused on the processing of the Japanese potential suffix. The subjects were 13 Japanese adults (mean age 27.1) and 13 advanced learners of Japanese as a second language (mean age 26.6). The production of the potential inflection by the two subject groups was compared under time pressure. The task stimuli were from the two morphophonological classes of Japanese verbs: vowel-stem verbs (verbs whose stems end with vowels) and consonant-stem verbs (whose stems end with consonants). The results indicated that native speakers were always more proficient than nonnative subjects, but for both groups, consonant-stem verbs were always more difficult than vowel-stem verbs. No interaction was found between the two variables, native-nonnative proficiency and verb classification. When the stimuli were further reclassified to reflect item difficulty, the distribution of errors was again found to be independent of whether the speaker was native or nonnative. It is concluded that within the area of morphophonological processing, nonnative speakers of Japanese closely resemble native speakers. (MSE)