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A taxonomic system was introduced by C. R. Chapman, D. Morrison, and B. Zellner [Icarus 25, 104–130 (1975)], in which minor planets are classified according to a few readily observable optical properties, independent of specific mineralogical interpretations. That taxonomy is here augmented to five classes, now precisely defined in terms of seven parameters obtained from polarimetry, spectrophotometry, radiometry, and UBV photometry of 523 objects. We classify 190 asteroids as type C, 141 as type S, 13 as type M, 3 as type E, and 3 as type R; 55 objects are shown to fall outside these five classes and are designated U (unclassifiable). For the remaining 118, the data exclude two or more types but are insufficient for unambiguous classification. Reliable diameters, from radiometry or polarimetry or else from albedos adopted as typical of the types, are listed for 396 objects. We also compare our taxonomy with other ones and discuss how classification efforts are related to the interpretation of asteroid mineralogies.