Teacher enthusiasm has long been considered an important part of the teaching process. However, empirical verification of enthusiasm as an indicator of teaching effectiveness is somewhat sparse. One problem is with measuring that complex variable, while another problem has been determining what to correlate it with to allow it to surface as such an indicator. Twenty-four preservice teachers participated in this study to determine the differences in teacher and student behavior between the levels of enthusiasm in trained and untrained teachers. The experimental group was given 6 hours of enthusiasm training whereas the control group received no such training. Both groups taught a 30-minute Experimental Teaching Unit (ETU) to a total of 120 middle-school students. The observation instrument in this study was the Physical Education Teaching Assessment Instrument (PETAI), while the Collins Enthusiasm Rating Scale was used to measure the teachers’ enthusiasm. The trained teachers received much higher ratings in enthusiasm during their ETU lessons and were significantly better on three of the PETAI items. The students of the trained teachers also had higher skill achievement gains over their counterparts under the untrained teachers.