Campylobacter jejuni is a major cause of infectious diarrhoea worldwide but relatively little is known about its ecology. In this study, we examined its interactions with Acanthamoeba castellanii, a protozoan suspected to serve as a reservoir for bacterial pathogens. We observed rapid degradation of intracellular C. jejuni in A. castellanii 5 h post gentamicin treatment at 25°C. Conversely, we found that A. castellanii promoted the extracellular growth of C. jejuni in co-cultures at 37°C in aerobic conditions. This growth-promoting effect did not require amoebae - bacteria contact. The growth rates observed with or without contact with amoeba were similar to those observed when C. jejuni was grown in microaerophilic conditions. Preconditioned media prepared with live or dead amoebae cultivated with or without C. jejuni did not promote the growth of C. jejuni in aerobic conditions. Interestingly, the dissolved oxygen levels of co-cultures with or without amoebae - bacteria contact were much lower than those observed with culture media or with C. jejuni alone incubated in aerobic conditions, and were comparable with levels obtained after 24 h of growth of C. jejuni under microaerophilic conditions. Our studies identified the depletion of dissolved oxygen by A. castellanii as the major contributor for the observed amoeba-mediated growth enhancement.