We present the development and validation of a science notebook rubric intended to measure the academic language and conceptual understanding of non-mainstream students, specifically fifth-grade male and female economically disadvantaged Hispanic English language learner (ELL) and African-American or Hispanic native English-speaking students. The science notebook rubric is based on two main constructs: academic language and conceptual understanding. The constructs are grounded in second-language acquisition theory and theories of writing and conceptual understanding. We established content validity and calculated reliability measures using G theory and percent agreement (for comparison) with a sample of approximately 144 unique science notebook entries and 432 data points. Results reveal sufficient reliability estimates, indicating that the instrument is promising for use in future research studies including science notebooks in classrooms with populations of economically disadvantaged Hispanic ELL and African-American or Hispanic native English-speaking students.