Despite very small gender differences in mathematic achievement tests, girls rate their mathematical abilities tower than boys as early as in elementary school. Based on the expectancy-value-model (Eccles & Wigfield, 2002), we tested if gender differences in mathematical ability self-concept are mediated by teacher and parent ratings of children's mathematical competencies as well as by different indicators of mathematical achievement. In a sample of N = 837 fourth graders, we assessed mathematical ability self-concept. teacher and parent ratings as well as mathematics grades and performance. Boys displayed a higher mathematical ability self-concept than girls. Objective mathematical achievement did not mediate the relationship between gender and mathematical ability self-concept, but grades in math, parent and teacher ratings did so partially. Results are discussed with regard to the importance of socializers and other potential factors impacting on the development of mathematical ability self-concepts.