In the field of educational psychology, there is diverse and active research in motivation for learning and achievement. Many instruments exist for assessing students' motivation, primarily as self-report. Fewer instruments are available for assessing "teachers'" perceptions of their students' motivation, and fewer still for assessing teachers' perceptions of "reasons for students' lack of motivation." Teachers' intervention strategies for motivation are linked to their causal perceptions. Therefore, it is important to assess those causal perceptions. In this paper, we offer evidence for the Perceptions of Student Motivation questionnaire, a new measure that offers evidence of validity and reliability for this purpose among high school teachers. It offers potential to increase efficiency and clarity of findings regarding teachers' perceptions of students' motivation. (Contains 7 tables.)