A computerized grammar checker was developed to assist teachers of English as a Second Language in editing student compositions. The first stage of development consisted of an error analysis of 125 writing samples collected from students. The 1,659 errors found were classified into 14 main types and 93 subtypes. This analysis served as the basis for constructing a taxonomy of mistakes and ranking the categories according to frequency of occurrence and comprehensibility. The grammar checker was then designed with a small electronic dictionary containing, 1,402 word stems and necessary features, and a suffix processor to accommodate morphosyntactic variants of each word stem. An augmented transition network parser equipped with phrase structure rules and error patterns was then constructed. In addition, a set of disambiguating rules for multiple word categories was designed to eliminate unlikely categories, increasing the parser's efficiency. The current implementation detects seven types of errors and provides corresponding feedback messages. Future research will focus on detecting more kinds of mistakes with greater precision and on providing appropriate editing strategies. (Author/MSE)