The importance of language to constituting meaning in science learning has been recognized. However, how language processes in learning are affected by the limited English proficiency of language minority students has not been addressed. The purpose of this study was twofold. The first goal was to describe how Mexican American language minority students construct biology concept meanings based on extant linguistic skills. Second, Vygotsky's semiotic approach to learning was used to design instructional activity to engage students in constructing meanings through mediational means such as science language, signs and symbols, and technology. Participants were 14 sophomores from an urban immigrant community who were enrolled in a Saturday enrichment program at a private university that maintained a "partnership" with the public high school. This interpretive study is based on extensive observational data, individual and group interviews, and students' written products. Findings indicated that students were overly reliant on the teacher's oral text as a source of science meaning and deferred to him for scientific interpretations of real life experience. Designed instructional activities showed students how to use semiotic tools to construct and express conceptual meanings. Diagrams were particularly important to instructional activity because these were modified to create visual parsimonious texts that were used as templates to support student appropriation of science language. An instructional sequence that moved students through three phases of learning was identified. Receptive understanding and expression used diagrams to identify content and enable students to "ventriloquate" teacher talk. Conceptual understanding and expression engaged students in constructing conceptual expressions and advanced students from "ventriloquating" teacher talk to expressing concepts for their own purpose. Interpretative understanding and expression was achieved by students using their conceptual understanding to analyze real life experience and their expanded discursive resources to write an interpretation. As students became proficient with semiotic tools, the teacher withdrew as science authority, and students assumed responsibility for constructing meaning using their own discursive resources. The findings demonstrate the importance of confronting the mystique of science through instructional activity that provides students the opportunity to acquire the cultural authority of language and other semiotic tools.