The notion of interlanguage is theoretically sound, and a number of recent articles have shown how error analysis can contribute to it by pinpointing those areas in which target language rules have been internalized, äs well äs those in which they have not. However, the characterization of transitional competence requires that the techniques of error analysis be applied to the oral and written performance of advanced learners so that we may identify those areas of linguistic competence which, even after several years' training, remain imperfectly mastered. This article embodies the results of an error analysis performed on a corpus of 61 papers written by 14 firstyear graduate students of Spanish at an American university. The results show that these advanced students still make errors of several types, particularly in the areas of morphology, syntactical construction, choice of prepositions, and choice of lexical items. Since in many cases such errors alternate with their correct counterparts, it is suggested that the formal analysis of transitional competence should u tilize variable rules of the type employed by Labov (1969Labov ( , 1970 in the study of linguistic variability among Speakers of substandard dialects of English.