A free-range broiler breeder flock was studied in order to determine the natural patterns of Campylobacter colonisation over a period of 63 weeks. Campylobacter STs were not mutually exclusive and on average colonised only 17.7% of the birds tested at any time. Campylobacter STs typically reached a peak in prevalence upon initial detection in the flock before tailing off, although the ST and antigenic flaA short variable region in combination were stable over a number of months. There was evidence that, with a couple of exceptions, the ecology of C. jejuni and C. coli differed, with the latter forming a more stable population. Despite being free-range, no newly colonising STs were detected over a 6 week period in autumn and a 10 week period in winter, towards the end of the study. There was limited evidence that those identified amongst broiler chicken flocks on the same farm site were likely to colonise the breeder flock earlier (R2 0.16, p 0.01). These results suggest that there is natural control of Campylobacter dynamics within a flock which could potentially be exploited in designing new intervention strategies, and that the two different species should perhaps be considered separately.