Two farrowing units housed 400 pigs each, and two fattening units housed 800 and 400, respectively; temperatures were controlled and the relative humidity was similar in each unit. Six-stage samplers collected microorganisms every 2 weeks for 6 visits from January to April; 4 and 20 minutes, respectively, were optimum for the total fraction and respirable fraction. The average airflow in the samplers was 28.3 litres/minute at 1 m above ground in the central aisle. The flow rate for dust sampling by filters was 2 m/minute for 1 hour. The total bacterial count was higher in the fattening houses. Averages of 48%, 15%, 52% and 19%, respectively, of total bacteria, gram-negative bacteria, fungi and yeasts of the farrowing units were in the respirable size range of < 5A