An investigation was made to determine the effect of an observation instrument on student teacher performance of Active Teaching Behaviors (ATB) focusing on how the teacher introduces, carries out, and concludes classroom instruction, and how the teacher maintains student engagement. Cooperating teachers were observed teaching mathematics and ranked as high or low in ATB. They then used the observation instrument in observing their student teachers teaching mathematics. They also shared the observation sheet with the student teacher and provided a set of definitions and examples of the behaviors to be observed. Results indicated that the use of an observation instrument in tandem with a cooperating teacher who displays high ATB may increase desirable teaching behaviors. Placement of a student teacher with a cooperating teacher with low levels of ATB who fails to use the observation instrument may result in a loss of desirable behaviors. (JD)