Eimeria tenella can cause the disease coccidiosis in chickens. The direct and often detrimental impact of this parasite on chicken health, welfare and productivity is well recognised, however less is known about the secondary effects infection may have on other gut pathogens. Campylobacter jejuni is the leading cause of human bacterial food-borne disease in many countries and has been demonstrated to exert negative effects on poultry welfare and production in some broiler lines. Previous studies have shown that concurrent Eimeria infection can influence colonisation and replication of bacteria such as Clostridium perfringens and Salmonella Typhimurium. Through a series of in vivo co-infection experiments, this study evaluated the impact that E. tenella infection had on C. jejuni colonisation of chickens, including the influence of variations in parasite dose and sampling time post-bacterial challenge. Co-infection with E. tenella resulted in a significant increase in C. jejuni colonisation in the caeca, in a parasite dose dependent manner, but a significant decrease in C. jejuni in the spleen and liver of chickens. Results were reproducible at three and ten day's post-bacterial infection. This work highlights that E. tenella not only has a direct impact on the health and well-being of chickens but can have secondary effects on important zoonotic pathogens.