Learners of English as a second language at intermediate and advanced stages have often mastered the majority of the major syntactic constructions in English. Yet, many grammatical errors persist in their writing. A high percentage of these errors, though labeled grammatical, do not in fact represent problems with pure syntax but rather mistakes in using given lexical items in constructions they do not belong in. By utilizing concepts from modern transformational-generative theories, the authors trace such errors to incorrect or incomplete lexical subcategorization. The nature of these errors is discussed from both a theoretical and pragmatic perspective, and the major classes of subcategorization errors for English verbs are identified. The article argues that ESL teachers, particularly writing teachers of students beyond the beginner’s level, need to be aware of the source of these errors so that they can distinguish them from other types of grammatical errors and more effectively help their students to overcome them.