Instruments for evaluating the implementation of inquiry science in K-12 classrooms are necessary if evaluators and researchers are to know the extent to which programs are implemented as intended and the extent to which inquiry science teaching accounts for student learning. For evaluators and researchers to be confident about the quality of these instruments, information about their development and validation--particularly about teacher self-report instruments--must be available. In this article, we present the Inquiry Science Implementation Scale, describe the instrument's development and the results of analyses of the validity and reliability of data collected with it, and discuss the possible uses of the instrument.