This is a study of Spanish in the San Diego area during the 19th century, before and after annexation of Alta California to the U.S. Evidence was obtained from seventy-two unpublished documents. The manuscripts were analyzed to establish their patterns of Spanish-English alternation and their phonological, morphological, syntactic, and lexical features. It is shown that Spanish among San Diego Californios retains vitality in public and private domains in the 19th century. The dialect is quite conservative and closely related to northern Mexican varieties. Moreover, it resists the influence of the English superstratum after annexation, with few innovations attributable to contact.