Observational bias can significantly affect results attained through observation. This study focused on 122 preservice teacher educators who conducted a structured observation, using momentary time sampling procedures with 10-second intervals, to measure student on-task and off-task behaviors. The experimental variable altered was the exceptionality label of the observed student to determine if the label would have a biasing effect. Labels used were oppositional defiant disorder, attention deficit hyperactivity disorder, and gifted/talented. Results suggest that an exceptionality label (i.e., oppositional defiant disorder) significantly affects observed behavior. Practical implications of the study are discussed. (Contains 2 tables.)