This is a discussion of another phase of bilingualism--structural difficulties in the target language of the student which impede his second language acquisition. The paper focuses on errors in learning English which do not derive from transfers from another language, and which cannot be predicted from contrastive analysis. These are intralingual and developmental errors; they reflect the learner's competence at a particular stage and illustrate some of the general characteristics of language acquisition. To best investigate and distinguish among interlanguage, intralingual, and developmental errors, the author studied speakers of varied languages; the diverse cross-section reduced greatly the possibility of mistaking intralingual errors for language interference. He concludes that an analysis of errors should lead to an examination of teaching materials and the underlying language learning assumptions. There is an appendix of typical intralingual and developmental errors. (FB)