The concentration of bioaerosols to which dairy farmers are exposed is potentially related to environmental factors, such as climatic conditions and individual management practices. An unprecedented heavy rainfall that was 250% of normal during the growing season of feed and bedding materials provided an unique opportunity for study. Individual dairy management practices differ as to barn construction, type of ventilation system, storage moisture of feed rations, quality of bedding materials, and animal density. The aim of this study was to identify the nvironmental factors affecting the concentrations of culturable bioaerosols in dairy barns. In this cross-sectional study of 48 dairy barns, area samples were collected using all-glass impingers. Culturable bioaerosols were analyzed to determine airborne concentrations of yeasts, molds, mesophilic bacteria, and thermophilic bacteria. The time-weighted geometric mean concentrations of these bioaerosols collected over the work-shift were 1.8x10⁴ cfu/m³ for yeasts, 0.8x10⁴ cfu/m³ for molds, 81.1x10⁴cfu/m³ for mesophilic bacteria, and 0.4x10⁴ cfu/m³ for thermophilic bacteria. These concentrations ranged from two to three orders of magnitude among the different barns. Bioaerosol concentrations did not differ between barns that used feed and bedding grown during extremely high rainfall and barns that used feed and bedding grown during normal rainfall. Multiple regression analyses were used to describe which environmental factors exhibited the strongest correlation with the concentration of bioaerosols. From these analyses, we conclude that efforts to reduce exposure to bioaerosols in dairy barns should focus on ventilation and storage moisture of feed rations.