Despite the importance of writing analytically in response to texts, there are few assessments measuring students' mastery of this skill. This manuscript describes the development of a response-to-text assessment (RTA) intended for use in research. In a subsequent validity investigation we examined whether the RTA distinguished among classrooms in students' ability to write analytically in response to text and whether measures of teaching predicted this variation. We demonstrate that the RTA was correlated with the state standardized assessment, but did not overlap with this accountability test completely and, additionally, that more variation between classrooms existed on the RTA. Students' opportunities for reasoning and extended writing in the classroom were significantly associated with RTA scores. The findings suggest that the RTA can be a valuable tool for conducting research on students' attainment of analytic writing skills and for understanding how teaching relates to student achievement on these skills. (Contains 12 notes, 6 tables, and 1 figure.)