The Language Profile of Bilingual Students in California was undertaken to determine: (1) students' range of performance on specific language tasks across four age groups in both Spanish and English; (2) the relationship of this performance to proficiency on two tests currently used in California; (3) how the development of the two languages compare; and (4) which task or combination best predicts language proficiency. The study involved 93 subjects, aged 5.0 to 9.11 years, in a transitional bilingual Spanish-English program. The language tasks assessed were sentence repetition, auditory association, story comprehension, naming in different categories, and story retelling. The study is reported in detail, including statistical data and texts of the tests used. It was found that the mean score for each task increased with age, except for hesitations and grammar errors, which were consistent across ages. Differences across languages were significant for a limited number of tasks, with better performance in English over time on tasks requiring less auditory verbal processing or paired with a visual stimulus. More tasks were significantly intercorrelated in English than in Spanish, which was attributed to the learning environments for the languages. The Language Assessment Scale currently used in the state for assessing proficiency was found to be limited in use for Spanish. Qualitative data also revealed that students used specific words reflective of their environment. Specific recommendations are made for proficiency testing based on these results, including consideration of students' stage of language development or possible loss of language, development of local norms, and followup studies. (MSE)