Fifty randomly selected fattening pig herds were studied to investigate the epidemiological characteristics of infections with Actinobacillus pleuropneumoniae serovars 2, 3 and 9, and to identify risk factors for their within-herd seroprevalences. Information about 13 farm characteristics was obtained by means of a questionnaire and used to assess potential risk factors for the percentage of slaughter pigs with antibodies against each of the three serovars. The presence of antibodies was measured with an indirect ELISA. The median within-herd seroprevalence for serovar 2 was 58 per cent (range 0 to 100 per cent), for serovar 3, 53 per cent (range 10 to 95 per cent), and for serovar 9, 35 per cent (range 5 to 100 per cent). All but one farm tested positive for A pleuropneumoniae serovar 2, and all the farms were positive for A pleuropneumoniae serovars 3 and 9. There was a positive association (P<0.05) between each pair of serovars. The within-herd seroprevalence of serovar 2 was significantly associated with the density of pig herds in the municipality (odds ratio [OR]=1.60; P<0-05) and with the absence of preventive medication at the start of the fattening period (OR=2.77; P<0.10). No significant risk factors were found for serovar 3. The percentage of pigs positive for serovar 9 was significantly associated with a slaughter date in June (OR=2.30; P<0.10) and with herds in which the finishing houses were not divided into separate compartments (OR=2.99; P<0.05).