Background: Boys earn lower grades in languages than girls. The expectancy-value model by Eccles" et al." (1983, "A series of books in psychology. Achievement and achievement motives. Psychological and sociological approaches," W.H. Freeman, San Francisco, CA, 76) is a comprehensive theoretical model for explaining gender differences in educational outcomes. In the past, most studies have focused on girls' disadvantage in math and science and on the role of the students' motivational beliefs. Aim: We aimed to explain boys' lower language grades by applying the expectancy-value model while taking into account students' motivational beliefs as well as their aptitude, prior achievement, and socializers' beliefs. In addition, we aimed at exploring the incremental contribution of each potential mediator. Samples: Five hundred and twenty German students (age M = 17 years; 58% female) and 374 parents (age M = 47 years). Methods: Student-reported ability self-concept (ASC) and task values, parents' perceptions of students' ability, students' prior achievement as reported by schools, and students' verbal intelligence test scores were all tested as mediators of the effect of gender on grades in German while controlling for parents' socioeconomic status. Single-mediator models and a multiple-mediator model were estimated using structural equation modelling. Results: All variables proved to be relevant for explaining boys' underachievement in language grades. Whereas students' ASC, task values, prior achievement, and parents' perceptions mediated the gender effect, verbal intelligence was identified as a suppressor variable increasing the gender effect. Conclusions: Our results challenge the stereotypic belief that boys' lower grades are due to lower verbal aptitude. Rather, students' motivational beliefs and parents' perceptions seem critical factors. Implications for both future research and practice are discussed.