1. A study was conducted to estimate the prevalence and quantification by species of Campylobacter infection in broiler flocks at the end of the rearing period and to identify associated risk factors. 2. A questionnaire about the rearing period was completed and caecal samples were collected from 121 broiler flocks in Brittany, France, during 2008. 3. Campylobacter was isolated in 87 out of 121 flocks--a prevalence of 71.9% (95% CI, 63.7-80.1%), including 40.5% of Campylobacter jejuni and 29.8% of Campylobacter coli. 4. The average concentration, in positive flocks, was 7.96 log10 cfu/g and ranged from 3.15 to 10.32 log10 cfu/g. 5. The average concentration by species was: 7.57 log10 cfu/g for C. jejuni and 8.44 log10 cfu/g for C. coli. 6. There was a seasonal effect, with increased risk of Campylobacter colonisation in June, July and August (odds ratio (OR) = 9.59, 95% CI 1.15-79.75). 7. The other factors, associated with lower risk of Campylobacter colonisation, were the acidification of drinking water (OR = 0.33, 95% CI 0.13-0.86), antibiotic treatment at the beginning of the rearing period (OR = 0.20, 95% CI 0.07-0.55) and rodent control around the house (OR = 0.18, 95% CI 0.03-0.95). 8. The results show that hygiene practices and biosecurity measures could lead to a reduction in Campylobacter colonisation.