Sex-related differential studies on mathematical abilities have hardly taken into account the mediator role of the verbal factor, which contrasts with the interest shown in the mediator role of visuospatial aptitude. We predicted that if sex-related differences were found, mental rotation would mediate mathematical abilities typically favoring males (geometry and verbal problems) and lexical access would mediate the one favoring females (arithmetic). Data from 455 participants with a median age of 13 were analyzed, showing that sex-related effect sizes in mathematical criteria were small, ranging from d=-.16 to .18 (corrected by attenuation), as expected for unselected samples. Lexical access scores were consistent predictors for every mathematical subdomain, and mental rotation added to the prediction only for geometry and word problems.