This pilot study describes elementary teachers' use of collaborative inquiry as a strategy for assessing Response to Instruction and Intervention (RtI [superscript 2]) in reading for an English Learner student. The design of the study was based on the sociocultural theory that assessment practices shape teachers' understanding of students and of the learning environment. Three case studies describe collaborative inquiry as a possible solution to the problem of inaccurate teacher interpretations when assessing English Learner (EL) students in English immersion programs. Inaccurate teacher assessments of EL students can lead to the continuation of insufficient instructional support and/or unwarranted referrals to the school's Student Study Team (SST). Research connects inaccuracies in teacher referral decisions to the over- and under-representation of EL students in special education. The accuracy and validity of teacher referrals and ultimately the instructional services for at-risk EL students depend on classroom teachers' analyses of EL students' individual differences in culture, second language acquisition and literacy development, as well as the classroom context for learning. An assessment of EL students' response to reading instruction that only examines students' component reading skills may not provide teachers with useful evidence about these students' unique experience of language and literacy in the classroom and at home. A sociocultural approach to RtI [superscript 2] potentially builds teacher capacity to use systematic and collaborative assessment practices that may benefit at-risk EL students by framing multiple sources of evidence that equip teachers to understand the unique needs of these students. In the current project an RtI [superscript 2] collaborative inquiry protocol was developed using the literature on Response to Intervention, school inquiry teams, sociocultural validity theory of assessment, and data based decision-making. Three teams composed of a lead teacher and two or three supporting teachers at three different urban, public elementary schools tested the protocol as a Tier I reading assessment strategy for EL students. Each lead teacher selected an at-risk EL student from her class to assess. The sample of eleven volunteer teachers assessed three target students during a five-week period in the spring of 2010. The RtI [superscript 2] collaborative inquiry protocol supported teachers with processes for data based, decision making within a problem-solving framework. The use of protocols guided teachers through an inquiry approach to data analysis that included: (a) delaying judgment by asking questions raised by the data; (b) collecting and analyzing multiple sources and types of data over a period of time; and (c) formulating hypotheses that linked probable, controllable causes of the student's learning problem with possible solutions. Teachers used evidence constructed from their analysis of the data to formulate and implement a Tier I plan for differentiating instruction and assessing the target students' response. Three case studies captured teachers' interpretations, decisions and actions, including their decision of whether or not the student needed additional, more intensive levels of support outside of the classroom. Evidence from direct observation, document review and interview data show that teachers' use of collaborative inquiry changed their assessment of the English Learner student and informed instructional decisions that created additional opportunities for the student to learn. Teachers' use of collaborative inquiry as an assessment strategy provided them with support, input and accountability that they used to implement a focused, individualized and responsive Tier I plan for the target student. Each lead teacher's plan addressed the student's academic and social-emotional needs and strengthened the home-school web of support for learning. Teachers said their new insight into the target student's needs and abilities encouraged them to continue targeted Tier I differentiation strategies. Teachers changed their attribution of the learning problem from external causes over which they had no control to problems of practice they could control. This was a surprising finding, given that these collaborative inquiry teams only worked together for three hours over five weeks. This result suggests that teachers' use of the collaborative inquiry protocol created a setting for them to address questions of practice similar to settings studied in the research on professional learning communities. A protocol for collaborative inquiry, such as the RtI [superscript 2] collaborative inquiry protocol tested in the current study may strengthen teachers' Tier I implementation in the classroom. Collaborative inquiry protocols may empower teachers to co-create the support and resources needed to solve instructional dilemmas without referring at-risk students out of the classroom. While the sample of teachers was small, the authentic description of teachers' work in actual school settings and from teachers' point of view supports the credibility of these findings. 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