**Background/Context:** Over the last two decades, school districts in the United States have increasingly allowed students and their families to choose the schools they attend and, at the high school level, the courses they take. While the movement to provide more curricular choice for students and families has accelerated, so, too, has the policy emphasis on increasing students’ math achievement. The increased emphases on curricular choice and math achievement provide an opportunity to examine how students draw on their social capital when making curricular choices and whether the diversity of their relational resources is associated with math achievement.

**Purpose:** We build from a social capital framework to examine how students who are able to exercise curricular choice do so by drawing on their social networks and how the resources accessible through these networks, operationalized as network diversity, are associated with math achievement. We also examine how this relationship varies by students’ math interest; an important individual-level characteristic that we hypothesize moderates the influence of network diversity on math achievement.

**Setting:** Data for this study are from the restricted-use version of the High School Longitudinal Survey of 2009 (HSLS: 09), the fifth in a series of National Center for Education Statistics’ multisource, secondary longitudinal studies. For this study, we rely on cross-sectional base-year data (2009) when all students were in Grade 9.

**Participants:** Our analytic sample consists of those students who: (1) were enrolled in and able to select their fall 2009 math course; (2) have valid scores on the dependent variable; and (3) have no missing values on items that constitute the independent variable-of-interest, network diversity. This subsampling strategy resulted in a final weighted, analytic sample of 5,570 students in 920 schools.

**Research Design:** Secondary analysis of cross-sectional observational survey data.

**Data Analysis:** Multilevel models with random intercepts are used to estimate students’ math achievement and properly adjust for the nested nature of the data. The models include controls for the HSLS stratified sampling design and for the probability of selection for individuals.

**Results:** After controlling for student- and school-level covariates, results indicate that our operational measure of social capital, network diversity, is significantly associated with math achievement. We also find that math interest significantly moderates this relationship, indicating that the presumed returns of social capital vary by this important non-cognitive characteristic.

**Conclusions:** Social capital in the form of network diversity helps all students reach resource- or information-rich contacts, such as teachers and counselors. However, by examining how math interest moderates the relation between network diversity and math achievement, we directly locate our work within an underappreciated theoretical niche that explicitly links how the presumed returns of social capital vary by student-level non-cognitive characteristics (e.g., math interest). Network diversity helps all students reach resource- or information-rich contacts including teachers and counselors. However, this does not guarantee that all students will see comparable returns. Results are further discussed in relation to schools’ curricular choice policies.