Heterogeneity is fundamental to learning and when leveraged in instruction, can benefit racially minoritized children. However, finding ways to leverage heterogeneity toward disciplinary teaching is a formidable challenge and teachers can benefit from targeted support to recognize heterogeneity in STEM, and its relationship to race and racism in disciplinary teaching. These data draw from a nine-day professional development seminar for secondary teachers to promote heterogeneity in STEM learning (n = 12). Drawing on analyses of lesson plans developed by teachers during the seminar, and subsequent video analyses of small group discussions, we present a case of four teachers debating heterogeneity in science. The exchange is significant because it draws into relief the ideological and emotional terrain of disturbing the racial hierarchy in which Western Modern Science (WMS) is steeped, and its implications for the education of racially minoritized youth. In the focus interaction, a dynamic emerged where three teachers exalted WMS, while the fourth grappled with how cultural heterogeneity has or could matter to her science teaching. Drawing on the constructs of racial-ideological micro-contestation and racial microaggressions, this analysis illustrates three important dimensions to the design of professional learning for STEM teachers that center race: (1) how discipline-specific discussions can uniquely surface the latent racial and ideological meanings teachers associate with STEM; (2) the centrality of teachers' storied knowledge in grappling with heterogeneity; and (3) the interplay of micro-contestation and microaggressions in understanding and anticipating the experiences of minoritized teachers when debating issues of race, disciplinarity, and teaching.