As a step toward developing appropriate instructional techniques to help learning disabled college students, mechanical errors were compared in the expository essays and controlled stimulus passages (rewrites) of 15 learning disabled, 15 normal, and 15 basic writers at the college level. Analysis of both types of writing samples showed that learning disabled students made the most errors in all categories: word usage, sentences, and spelling. While learning disabled students' most frequent errors were omitted words, dropped endings, and omitted salient information, the most common errors by basic writers were comma omissions, faulty parallelism, and sentence fragments. The differences in error types might imply an underlying processing problem for learning disabled students and a lack of instruction for the basic writers. Differences in error patterns between the controlled stimulus passage and expository essay were also evident. When producing text in a free writing situation, for example, learning disabled students created fewer meaningless sentences but made more errors with demonstratives and prepositions. (MM)