Based on the Eccles' expectancy-value theory, the objectives of this study were (1) to examine the longitudinal relations between Latino parents' science beliefs, parents' science support, and high school students' science motivational beliefs, and (2) to test whether these relations varied by familism values and adolescent gender. Multi-informant longitudinal survey data were collected from 104 Latino parents and adolescents during 9th and 10th grade. Parents' perceptions of their adolescents' science ability and value of science in 9th grade predicted higher parent science support at home in 10th grade if they were parents of boys. In contrast, these relations were not statistically significant for parents of daughters. Parent support in 9th grade predicted higher adolescent motivational beliefs one year later. This work provides evidence that parent support is important for high school student success in this understudied population.