Use of a live oocyst vaccine in broilers has become a widely accepted means of controlling coccidiosis. The commonly used live oocyst vaccines can be administered to chicks at hatch using spray cabinets or to late stageembryos by in ovo injection (Williams, 2007). The live nonattenuated vaccine, Inovocox EM1, is recommendedfor the immunization of healthy broiler embryos between 18.0 and 19.0 doi for the prevention of coccidiosis. Therefore, the accuracy of injection, the movement of oocysts within the embryo, and their subsequent effects on broiler embryogenesis and hatching chick quality were investigated in eggs that were set and injected 12 h apart under the same incubation conditions A previous report by Sokale et al. (2017a) showed the importance of ES in accurately determining embryo physiological development. In the current study, embryos in the 19.0 IAN group had a higher ES incomparison to those in the 18.5 IAN group. This in dicates that at the time of in ovo injection, there weredevelopmental differences between the 2 groups, with embryos in the 19.0 IAN group showing more advancedphysiological developmental characteristics. These advancements were mostly recognized in the positioning of the head and the occurrence of pips through the eggshell membrane and the eggshell prope