Campylobacter bacteria are an important cause of bacterial gastro-enteritis in man. Although food of animal origin is the main source of human infection, a casecontrol study in the United States of America showed that 8% of all campylobacteriosis cases could be attributed to consumption of contaminated surface water. In this thesis the prevalence of Campylobacter in sewage purification plants was investigated in order to obtain more information on the survival of this pathogen in aquatic environments. A survey carried out on three municipal plants showed that sewage and surface waters are frequently contaminated with Campylobacter. The contamination of sewage was higher when meat-processing industries were present in the drainage area of these plants. Indeed, drain water of a poultry abattoir contained high numbers of Campylobacter . It is clear that other sources contribute far lower numbers to sewage. Furthermore, the aquatic Campylobacter isolates were more resistant to quinolones and to ampicillin when a meat-processing industry was draining its waste on the sewerage.The purification process reduced the numbers of Campylobacter , but this pathogen was not eliminated completely. The prevalence and reduction were not correlated with enviromnental parameters, such as water temperature and oxygen pressure.The phenomenon of the transformation of spiral Campylobacter cells to coccoid cells was also investigated. Determination of several physiological parameters indicated that the contribution of the nonculturable, coccoid Campylobacter cells is to infection routes probably negligible.Considering the observed occurrence of Campylobacter in surface waters, in the context of the reported dose-response model for this pathogen, it can be concluded that the role of surface waters in the epidemiology of Campylobacter may be underestimated.