Knowledge of derivational morphology can aid readers in the analysis and acquisition of new vocabulary, in lexical access, and in establishing the syntactic structure of sentences. A study investigated good and poor readers' knowledge and use of derivational suffixes in establishing sentence level syntax. Subjects, 123 10th and 11th grade students, completed a vocabulary checklist test, a comprehension test, and a direct test of morphological knowledge. Results indicated that knowledge and use of derivational morphology was correlated with reading ability. However, for both good and poor readers, the use of knowledge about morphological structure of complex forms was task specific. Although the students possessed knowledge of the syntactic properties of derivational suffixes, even the above average readers did not appear to use such knowledge in a task that required reading for meaning. (Author/FL)