High levels of airborne particulate matter (PM) and bacteria may exist in animal housing, which can be detrimental to the health of animals and workers. The sizes of these bioaerosols determine their aerial-transport behaviors and depositions in the respiratory tracts of animals and humans. However, little is known regarding the size distribution of airborne PM and bacteria in livestock houses, especially alternative animal housing systems that aim to enhance animal welfare, such as aviary hen-housing systems. The study reported here was therefore conducted to characterize the concentrations (both in count and in mass) and size distributions of airborne bacteria (in count) and PM (both in count and in mass) in a pilot-scale welfare-oriented aviary laying-hen setting. Thirty-four laying hens were kept in the environmentally-controlled aviary setting (L x W x H = 2.2 x 2.3 x 2.4 m) for 3 months. The hens were given a 16L:8D photoperiod (lights on at 6:00h and off at 22:00h); and access to the litter floor from 12.00h to 22:00h daily. Airborne bacteria and PM were simultaneously sampled for 15 min at 1.5 m height above the litter floor every fourth day at 5:45h, 9:45h, 13:45h, 17:45h, and 21:45h. Concentrations of airborne bacteria at six size ranges (0.65-1.1 μm, 1.1-2.1 μm, 2.1-3.3 μm, 3.3-4.7 μm, 4.7-7.1 μm, and >7.1 μm) and the PM concentrations (0.5-20 μm) were determined. The daily mean (±SD) concentrations of PM count, PM mass and total bacterial concentration were, respectively, 1.70 (± 0.66) ×107 particles m -3, 1.12 (± 0.47) mg m-3 and 3.39 (± 2.38) ×105 CFU m-3. Concentrations of PM and total bacteria during the litter-access period (12:00h-22:00h) were significantly higher than those during the rest of the day when the hens were off the floor (P <0.05). Median diameters for the PM count and mass were, respectively, 2.11 μm and 7.45 μm. PM <10 μm accounted for more than 95% of the total PM count, whereas PM >2.5 μm accounted for more than 90% of the total PM mass. The majority (>95%) of the airborne bacteria were contained in particles >3.3 μm. Airborne bacteria count was positively related to PM mass concentration (P <0.05) with a slope of 3.84(± 2.70) ×705 CFU mg-1 PM. Results of the study are useful for improving understanding of transport behaviors of aerosols in the aviary hen setting, assessing potential respiratory risks to humans and animals, and exploring mitigation techniques.