The contribution of primary biological aerosol particles (PBAP) to the global budget of organic aerosol is poorly understood. Concentrations of mannitol, a biotracer for fungal spores, are used here to constrain the first global model (GEOS-Chem) simulation of PBAP from fungi. Emissions are driven by leaf area index and atmospheric water vapor concentrations and are empirically optimized based on the geographical and seasonal variability of observed mannitol concentrations. Optimized global emissions total 28 Tg yr−1, with 25% of that total emitted as fine mode (PM2.5) aerosol. Fungal spores contribute 23% of total primary emissions of organic aerosol, or 7% of the fine-mode source. Annual mean simulated surface concentrations of PBAP over vegetated regions range from 0.1–0.7 μgm−3 (PM2.5) and 0.4–3.0 μgm−3 (PM10), with the highest concentrations in the tropics, where PBAP may be the dominant source of organic aerosol. Further validation is required to reduce the substantial uncertainties on this budget.