Ensuring that new forms of assessment are fair and valid for language minority students is essential for research on assessing the performance of Latino language minority elementary school students engaged in a cooperative learning language arts curriculum in Spanish and English. Strategies for developing performance assessments are described, and results from a preliminary study implementing these strategies as part of the curriculum for 39 bilingual and monolingual third graders are presented. The research shows how the design of individualized performance assessments might be devised based on ethnographic observation of the children's interaction and goals for cooperative learning established by the teacher. How analysis of children's interaction in cooperative learning can show "in situ" classroom assessments among the children that help validate interpretation of performance is also described. Three appendixes contain study questions, charts of change, and transcript conventions. (Contains 11 references.) (Author/SLD)