Scholars have documented that the masculine work cultures characteristic of academic science, technology, engineering, and mathematics (STEM) disciplines create an unwelcome climate for women. One way academic STEM cultures can negatively impact women, especially mothers, is by shaping how hard they think they have to work on the job–a perception that can affect job satisfaction, stress levels, and attrition. This study assesses faculty member’s reports of how hard they have to work on the job and the extent to which their sex, parental status, and academic discipline shape these reports. Survey data from roughly 300 tenure-line faculty members in a research university find that mothers in STEM disciplines perceive that they have to work harder than STEM and non-STEM fathers and mothers not in STEM disciplines. These differences remain net of controls for institutional tenure, rank, perceived job demands, work-family overlap, and time spent on job and family responsibilities.& I interpret findings as evidence that STEM mothers encounter challenges to their competence–as mothers and as female experts in STEM disciplines with masculine cultures–that make them feel they must work harder than others.& I discuss implications of findings for the experiences of women in academic STEM disciplines and policy reform intended to shift these perceptions.