The validity of teacher locus of control (TLOC) as measured by an instrument adapted for this purpose was studied. The construct refers to the individual's perception that changes in his or her environment result from internal or external factors. An adaptation of a scale developed by R. C. Taylor et al. (1981) was used to measure TLOC. The first study investigated the reliability of the scale using data from 54 primary and secondary teachers (35 female and 19 male) in a course on classroom-based research. The external criterion was performance ratings of these teachers by their supervisors. Discriminant analysis of the instrument indicated its efficacy in differentiating internals from externals. The second study tested the hypothesis that more effective teachers would be more internal as compared with less effective colleagues. Thirty female and 10 male teachers from one secondary school completed the TLOC instrument. They were separated into effective and less effective teachers by principals' ratings. Teachers rated as more effective were in fact more internal. To the extent that supervisors' evaluations are reliable and valid measures of teacher performance, locus of control can be considered to have predictive validity. Four tables present study data. (SLD)