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This study examines the computer-based (PC) version of the Mochizuki Vocabulary Size Test and compares it to its written version. Three forms of the PC version and one form of the written version were conducted among 72 Japanese learners of English, while one form of the PC version was administered to 110 learners. Test scores and questionnaire responses were analyzed through the use of multiple methods, including Rasch analysis. The results suggested that (a) the test items spread moderately, with few gaps, on the Rasch scale of item difficulty, (b) there were few problematic items, (c) the difficulty generally increased at higher levels, (d) test-takers felt that the PC and written versions shared the construct of vocabulary knowledge/ability but differed in that the PC version measured speed of lexical retrieval, due to its imposed time limits, (e) the items had unidimensionality, high reliabilities, and measurement invariance, (f) three forms of the PC version and one form of the written version are parallel, and (g) that there were strong correlations among the four forms and between each form and the Test of English for International Communication (TOEIC^[○!R]). In conclusion, all results were deemed positive and useful in producing a validity argument.