The mothers of 29 Spanish English bilingual 25-month-olds kept diary records of their children's dual language exposure and provided information on their children's English and Spanish language development using the MacArthur-Bates inventories. Relative amount of exposure predicted language outcomes in English and Spanish. In addition, the number of different speakers from whom the children heard English and the percent of their English input that was provided by native speakers were unique sources of variance in children's English skills. These properties of children's dual language exposure and their bilingual proficiency varied as a function of whether the children's mother, father, or both parents were native Spanish speakers. Practical and theoretical implications are discussed.