Charles A. Ferguson's concept of "diglossia" (1959, 1964) is used in analyzing sign language. As in Haitian Creole or Swiss German, "two or more varieties" of sign language are "used by the same speakers under different conditions"--these are here called "High" (H) sign language and "Low" (L) sign language. H sign language is formally taught "Manual English," English words fingerspelled or signed. L is the less prestigious form which is used, for example, in communicating with children or in informal conversations. As in other cases of diglossia, speakers may not be aware of, or may even deny the existence of, two distinct varieties, preferring to believe they use only H. Also as in other forms of diglossia, sign language L is simpler grammatically than H. The bulk of vocabulary of H and L is shared, H having more technical terms and L often being used to explain an idea "in words of one syllable." In applying the phonological analysis of diglossia to sign language, it is necessary to ask if H and L constitute a single "gestemic system" and if this system is common not only to users of sign language but all human communicators. It is concluded that sign language diglossia is a special situation but relatively stable and that this condition should be recognized in teaching sign language to the deaf. The author invites correspondence on this issue, c/o Department of English, Gallaudet College, Florida Ave. and 7th, N.E., Washington, D.C. 20002. (JD)