This study reports on the process of developing a test to assess students' reading comprehension of scientific materials and on the statistical results of the verification study. A combination of classic test theory and item response theory approaches was used to analyze the assessment data from a verification study. Data analysis indicates the test has satisfactory validity and reliability. The Reading Comprehension of Science Test (RCST) components have a wide range of difficulty, which suggests that the RCST is appropriate for a relatively large percentage of students. Based on the accepted relationships among science, language, and literacy, integrated literacy-science instructions are highly recommended.