An experiment was conducted on the effects of two teacher variables and student engagement on science achievement. The variables included: teacher wait-time, defined as the length of the pause preceding a teacher utterance; and questioning quality, a generic term used to describe cognitive level, clarity, and relevance of questioning. Thirteen middle school teachers were randomly assigned to four groups to receive feedback on: wait-time and questioning quality; wait-time only; questioning quality only; and placebo feedback only. The feedback was designed to increase teacher wait-time beyond three seconds and to increase the quality of teacher questioning. Twelve students were selected from each class in order to estimate rates on nine different categories of engagement. Measures of formal reasoning ability and locus of control were obtained for all students in the study. Variation in science achievement was significantly related to variation in teacher wait-time, formal reasoning ability, and two categories of student engagement (attending and generalizing). The results indicated that achievement may be increased by using a mean teacher wait-time of approximately three seconds and ensuring that students are maximally engaged on the instructional objectives. (Author/JN)