Girls often express less interest in STEM (science, technology, engineering, mathematics) education and careers than boys, despite having comparable aptitude. We randomly assigned 242 girls (Mdn[subscript age] = 12 years; 38% East Asian; 37% White) at Canadian STEM camps to control conversations about generic camp experiences, or intervention conversations where STEM role models discussed how STEM education and careers align with each girl's most important value and emphasized the social community inside and outside of STEM. Key measures were collected at baseline, and several days after the intervention (or control). Girls' "current" STEM fit did not differ by condition, but as expected, the intervention (vs. control) significantly improved girls' forecasts regarding "future" STEM fit (ds = 0.27-0.35) and girls' interest in STEM careers (d = 0.42), with a marginally significant boost in girls' interest in STEM high school classes (d = 0.23). Pre-post increases in forecasted STEM fit mediated increases in STEM interest. Forecasted fit (beyond current fit) appears pivotal for promoting girls' sustained interest in STEM.