The relationship between expert teachers' classroom behavior and their subject matter knowledge is an area of research that has not been examined extensively. To begin that study, one topic, fraction knowledge, was explored in depth as it occurred in natural teaching settings. Fractions are one of the more difficult topics in elementary arithmetic; much of this difficulty is attributable to the complex relationships among the meanings and representations of fractions and basic arithmetic operations. Despite these difficulties associated with fractions, many teachers are quite proficient at teaching children to perform operations with fractions. The content and organization of expert teachers' knowledge of fractions was investigated using interviews, card sorting tasks, and transcriptions of videotaped lessons. Semantic networks that reflected their knowledge of fractions were developed for individual teachers. Comparisons of these semantic networks showed that there were wide disparities among the knowledge of expert teachers. Some teachers displayed relatively rich conceptual knowledge of fractions while others relied upon precise knowledge of algorithms. Implications of these knowledge differences are discussed. (Author/JN)