This paper explores Mexican-American prospective teachers' use of culture--defined as social practices and shared experiences--as an instructional resource in mathematics. The setting is an after-school mathematics program for the children of Mexican heritage. Qualitative analysis of the prospective teachers' and children's interactions reveals that the nature of the mathematical activities affected how culture was used. When working on the "binder activities," prospective teachers used culture only in non-mathematical contexts. When working on the "recipes project," however, culture was used as a resource in mathematical contexts. Implications for the mathematics teacher preparation of Latinas/os are discussed.