Individuals are at an increased risk to drop out of the STEM pipeline if they are female or Latino, and during certain periods including high school. Families are a potential untapped resource of support for high school students. Based on the expectancy-value model, we examined if a variety of parental behaviors predicted students' ability self-concepts in and value they placed on biology, chemistry, and physics. Self-report surveys were collected from 988 9th grade Latino boys, Latina girls, Caucasian boys, and Caucasian girls. The findings suggest that, as early as the beginning of high school, students hold different motivational beliefs for biology, chemistry, and physics. Caucasian boys reported higher parental behaviors and motivational beliefs compared to Latino boys, Latina girls, and Caucasian girls. Latina girls reported the lowest parental behaviors and motivational beliefs. Parent education and Spanish language use partially explained some of these differences suggesting ethnic differences are in part due to differences among Caucasians and Latinos on parent education and language use. Parents' positivity, co-activity and school-focused behaviors predicted higher adolescent ability self-concepts and importance values in all three sciences for all adolescents in this study. Parents can support adolescents in science through a variety of behaviors at home. Many of these behaviors do not require parents to be science experts and thus may be attainable for a range of families.