A progress report is presented of a study which applies a system of generative grammar to error analysis. The objective of the study was to reconstruct the grammar of students' interlanguage, using a systematic analysis of errors. (Interlanguage refers to the linguistic competence of a student who possesses a relatively systematic body of rules, without complete mastery of the target language.) Specifically, the research studies the way a foreign student learns French interrogative structures. The report covers the following areas: (1) a description of the system of generative grammar as applied to the syntax of simple French interrogative sentences and as used in this research; (2) syntagmatic rules; (3) transformational rules; and (4) experimental application of the reference grammar that was elaborated. In line with the primarily pedagogical objective, the experimental material was an oral test on simple French interrogatives recorded by students on the level of "francais fondamental." To illustrate the process used, the method of analyzing one student's errors is described. (AMH)