Effective teacher beliefs about students are an integral part of effective teaching. Teachers with interventionist beliefs about students ("I can intervene to help a learner with difficulties") show more effective practice than teachers with pathognomonic beliefs ("I blame the learner for his difficulties"). A professional development (PD) course sensitized teachers (N = 234) to individual learning differences (ILDs), using five learning/cognitive styles tools. Teachers' responses to a pre-/post-test question concerning their beliefs about "weak students" were analyzed and correlated with their ILD scores. Before the PD, teachers with strong ILD preferences matched to traditional learning contexts were significantly more "at risk" (i.e., had fewer interventionist beliefs) than the other teachers; the former teachers were significantly overrepresented in the sample. After the PD, teachers' interventionist beliefs significantly increased, regardless of their ILD preferences. Neither the length of the PD (28 hrs. vs. 56 hrs.) nor the amount of teaching experience affected the teachers' interventionist beliefs about students. A mediated, constructivist and collaborative PD, which sensitizes teachers to individual learning differences, can increase effective teacher beliefs about students. We conclude that developing more effective teacher beliefs about learners should become a component of teacher professional development. (Contains 11 tables and 8 figures.)