Campylobacteriosis is currently the most frequent foodborne zoonosis in many countries. One main source is poultry. The aim of this study was to enhance the knowledge about the potential of bacteriophages in reducing colonization of broilers with Campylobacter , as there are only a few in vivo studies published. Commercial broilers were inoculated with 10⁴ CFU/bird of a Campylobacter jejuni field strain. Groups of 88 birds each were subsequently treated with a single phage or a four-phage cocktail (10⁷ PFU/bird in CaCO₃ buffered SM-Buffer). Control birds received the solvent only. Afterwards, subgroups of eleven birds each were examined for their loads with phages and Campylobacter on day 1, 3, 7, 14, 21, 28, 35 and 42 after phage application. The susceptibility of the Campylobacter population to phage infection was determined using ten isolates per bird. In total 4180 re-isolates were examined. The study demonstrated that the deployed phages persisted over the whole investigation period. The Campylobacter load was permanently reduced by the phage-cocktail as well as by the single phage. The reduction was significant between one and four weeks after treatment and reached a maximum of log₁₀ 2.8 CFU/g cecal contents. Phage resistance rates of initially up to 43% in the single phage treated group and 24% in the cocktail treated group later stabilized at low levels. The occurrence of phage resistance influenced but did not override the Campylobacter reducing effect. Regarding the reduction potential, the cocktail treatment had only a small advantage over the singe phage treatment directly after phage administration. However, the cocktail moderated and delayed the emergence of phage resistance.