Colibacillosis is the main bacterial disease in poultry and is treated mainly by antibiotics, which eventually led to the emergence of antibiotic resistant strains. Our objective was to evaluate the efficacy of two phage cocktails to prevent colibacillosis induced in chicks by the pathogenic strain BEN4358. The first cocktail was constituted of eight phages of different genus, four phages were able to replicate on BEN4358, leading to the death of the bacterial cells; three phages were not able to replicate on the strain, but were able to lyse bacterial cells by the action of virion-associated enzymes and one phage was not active on BEN4358. The second cocktail was composed of the four phages able to replicate on BEN4358. In a chicken embryo lethality assay, the cocktail of 8 phages allowed 90% of chicken embryos to survive an infection by BEN4358, in contrast to the control (BEN4358 only), which gave a survival rate of 0%. Then, embryonated chicken eggs were inoculated by the phage cocktails in the allantoic fluid at 16 days. At day 19, embryonated eggs were transferred from the egg incubator to isolators, where the chicks hatched. Strain BEN4358 was inoculated subcutaneously at one-day-old chicks. Even if phages were detected in chicks, the mortality between the non-treated and the phage-treated groups was 75% and 80%, respectively. There was no reduction in BEN4358 intestinal load. Same in vivo experimental scheme was conducted with the second cocktail and the chicks were challenged with a 10-fold reduced bacterial inoculum. There was no difference in the mortality rate between both groups; however, the mortality was delayed in the treated-group. Moreover, the intestinal BEN4358 load was statistically significantly lower in the phage-treated group (2.2x107 CFU/mL) compared to the non-treated group (1.3x108 CFU/mL).Thus, in ovo inoculated phage cocktails did not prevent colibacillosis but phages were transmitted to chicks and one cocktail reduced intestinal carriage.