Emerging antibiotic-resistant bacteria has been explained as a critical concern in medicine and veterinary medicine recently. Also transferring the antibiotic resistance genes from animals to humans is confirmed previously. Therefore, finding new and effective alternatives to antibiotics is inevitable. Bacteriophages (phage therapy) was successfully used to remove and treatment of different infections in humans and animals. In this study 50 antibiotic resistant isolates of E.coli from clinical cases of poultry colibacillosis were isolated and confirmed using 16srRNA gene detection in East Azerbaijan, Iran. A lytic bacteriophage was isolated against the most resistant pathogenic isolate as host using agar bilayer method. Host range of the bacteriophage was evaluated against all used isolates and then, its morphological and biological features was determined. In experimental assays, antibacterial activity of the bacteriophage was evaluated in different groups of broiler chickens comparing with ciprofloxacin. According to the results, the phage was classified in Podoviridae family and it was named PEcMa6/19. The phage showed stability in temperature range of -80 to 60 ℃.It can survive in pH rate of 2-12 but it was sensitive to ethanol, DMSO, acetone and SDS. In experimental phase, in antibiotic treatment groups immediately, 24 h and 48 h after bacterial injection, rate of mortality among chickens were 20, 16.6 and 23.3 % while in phage treatment groups theses rates were determined 6.6, 13.3 and 13.3% respectively. Our results confirmed the high antibacterial potential of bacteriophage PEcMa6/19 to control of colibacillosis but for expanded application in poultry flocks molecular analysis of the bacteriophage is needed
