Based on a sample of 312 high school teachers who participated in the Israeli PISA assessment of student academic achievement in 2002, the current study examines the mediatory role of their perception of the subject matter (as "open/flexible'" or "closed/hierarchical") in the relation between their disciplinary expertise (language, mathematics or science) and their grading style (performance-output or effort-input). The mediation hypothesis was completely supported for all disciplines in the case of perception of subject matter as open/flexible. With respect to the closed/hierarchical perception, it was supported only for the comparison of science vs. mathematics.