The effect of an experimental Eimeria tenella invasion upon an artificial Salmonella Typhimurium infection was studied in 200 broiler-chickens. The birds were divided into 5 experimental groups. The first group was challenged with S. Typhimurium for three consecutive days. By the 4th day, the chickens were infected once with sporulated E. tenella oocysts. The second group was infected once with E. tenella and 24 h later infected with S. Typhimurium during three consecutive days. The third group of birds was submitted to a single E. tenella infection. The birds from group IV were infected with Salmonella for three days whereas chickens from group V were not infected (negative controls). The highest values of parameters, determining the severity of the epidemy (morbidity rate, lethality, cumulative mortality) were observed in the group where the Eimeria invasion preceded the Salmonella infection.The most frequent and most prolonged isolation of Salmonella from parenchymal organs was also observed with birds with mixed infection, with Eimeria infection coming first before salmonellosis. The same group was characterized with the highest values of oocyst and lesion indexes as well as with the worst economic results lowest live body weight and lowest daily weight gain.