This study investigated the relative importance of three subdomains of early number competence (number, number relations, and number operations) in predicting later mathematics achievement in cross-sequential samples of pre-kindergarten (pre-K), kindergarten, and first-grade children (n = 150 at each grade). Ordinary least-squares (OLS) regression analyses showed that each subdomain predicted mathematics achievement at each grade level, controlling for the other two subdomains as well as background variables. All the subdomains explained a significant amount of variance in later mathematics achievement. Unconditional quantile regression analyses examined relations between number competencies and mathematics achievement at quantiles representing low (0.2), intermediate (0.5) and high (0.8) achievement. The subdomain of number operations was highly related to mathematics achievement for high achievers. For low achievers, future mathematics achievement was most highly related to number and number relations abilities in the pre-K sample and to number relations abilities in the kindergarten and first-grade samples. Findings highlight the unique importance of all three subdomains of early number competence for later mathematics achievement, but they show that some of the relations are contingent on achievement level. (author abstract)