Stream warming alters fungal community dynamics and growth rates on leaf litter, but its effects are not well understood. This study utilizes quantitative stable isotope probing (qSIP) and an artificial stream mesocosm system to investigate fungal responses across three temperature treatments. Sycamore leaves were placed in 48 chambers and harvested at weeks 1 and 7 to assess successional changes in fungal species composition and growth rates. At each harvest, leaf discs were cut and incubated for one week in ¹⁸O-enriched water. Isotope assimilation into fungal DNA was used to quantify relative abundance and taxon-specific growth rates. Fungal species composition varied across temperature treatments with most differentiation occurring in rare fungal species. These findings suggest that temperature changes may disproportionately impact the less common members of fungal communities. This research enhances our understanding of how stream warming reshapes fungal communities, potentially disrupting ecosystem functions and nutrient cycling, with the added potential for amplification due to climate change.