It has long been noted in research literature that there does not exist a shared definition of integration of science and mathematics. Classifying a particular set of integration practices by one of many labels has limited value. Moving past the definition debate, this study describes the development and testing of the Mathematics Integrated into Science: Classroom Observation Protocol (MISCOP). The MISCOP consists of 20 items separated into five constructs. The instrument was found to be internally consistent and has strong construct-to-total correlations. Factor analyses revealed five factors that, while not completely aligned with the five designed categories, did have notable correspondence. The pilot testing and analyses reveal the MISCOP to be a useful instrument for measuring the degree to which mathematics is integrated into student-centered learning of science.