This article reports on the development of an online dynamic approach for assessing and improving students' reading comprehension of science texts--the dynamic assessment for reading comprehension of science text (DARCST). The DARCST blended assessment and response-specific instruction into a holistic learning task for grades 5 and 6 students. The DARCST, a layer-cake approach (interspersed responses and feedback) to assessment as learning, takes a graduated-prompts perspective on the criteria for reading comprehension of science text. The item response theory (IRT) analysis and analysis of variance (ANOVA) over an instructional period indicated that the DARCST was applicable for most students with a wide range of reading abilities. Based on the acceptable psychometrics, the DARCST appears to be appropriate for use in middle-years science programs where enhanced science reading is a goal.