Knowledge of morphology--the ability to gain information about the meaning, pronunciation, and part of speech of new words from their prefixes, roots, and suffixes--is an important component of skilled reading. As part of a larger program of research on morphology, a study explored the development of students' knowledge of common English suffixes. Two aspects of knowledge about suffixes were investigated: (1) the ability to use the suffix on a new word to determine that word's part of speech and its appropriate use in a sentence; and (2) the ability to distinguish between true suffixed words (e.g., "swimmer") and nonsuffixed words that contain the orthographic form (but not the meaning) of a root word and suffix (e.g., "mother"). A battery of tasks measuring knowledge of morphology was administered to 720 fourth-grade, seventh-grade, and high school students. Knowledge of common English suffixes was found to continue to develop after fourth grade; even in high school, some students had serious problems with these suffixes. Knowledge of morphology appears to be a distinct component of verbal ability, although it is significantly related to standardized measures of reading ability in seventh grade and high school. The instruments developed in the study identified students who have particular difficulties with English suffixes, and thus have potential as diagnostic tools. (Author)