A theoryof interlanguage(IL)lexiconsis outlined,with emphasis on IL lexical entries, based on the HPSG notion of lexical sign. This theory accounts for idiosyncratic or lexical transfer of syntactic subcategorisationand idioms fromthe firstlanguage to the IL. It also accounts for developmental stages in IL lexicalgrammar,and grammaticalvariationin the use of the same lexical item. The theory offers a tool for robust parsing of lexical transfererrors and diagnosisof sucherrors.