Research has shown causal and correlational links between teacher enthusiasm and student achievement, and between student at-task behavior and student achievement. This study tested the causal link between teacher enthusiasm and student achievement, using teaching as an independent variable and at-task behavior as a dependent variable in mathematics classes for students from the first to sixth grades. An experimental group of teachers participated in a two-week training procedure for enthusiasm. Experimental and control group teachers were videotaped both before and after training with observers recording pupils' at-task behaviors on a five-second interval system. The results showed that the teachers who had enthusiasm training showed a significant increase in level of enthusiasm, and that their pupils had a significantly higher level of at-task performance, under both direct and indirect teacher influence. (FG)