The objective of this study was to investigate the effect of eggshell temperature (EST) and carbon dioxide (CO2) concentration during only the hatching phase on embryonic development and chick quality. Three batches of eggs were incubated at an EST of 37.8°C until d of incubation (E) 19. From E19, embryos were incubated at low (36.7°C), normal (37.8°C), or high (38.9°C) EST and at low (0.2%) or high (1%) CO2 concentration. Organ growth and embryo and chick quality were measured at E19, internal pipping (IP), hatch, and 12 h after hatch. A few interactions between EST and CO2 were found at IP, hatch, and 12 h after hatch, but all of these interactions were temporary and in most cases weak. High EST resulted in a lower relative heart weight compared with low ( = 0.05) and normal EST ( = 0.06) at IP, compared with low ( = 0.11) and normal EST ( = 0.08) at hatch, and compared with low ( = 0.11) and normal EST ( = 0.08) at 12 h after hatch. At hatch, high EST resulted in a lower YFBM compared with low EST ( = 0.65). At 12 h after hatch, high EST resulted in a lower relative liver weight compared with low EST ( = 0.12). At low EST, greater relative intestinal weight was found compared with normal ( = 0.41) and high EST ( = 0.37). The effect of CO2 solely was found at 12 h after hatch at which a higher relative heart weight ( = 0.05) and a higher relative lung weight ( = 0.0542) was found at high CO2 compared with low CO2. High EST during only the hatching phase negatively affected chick development, mainly expressed by the lower relative heart weight at IP, hatch, and 12 h after hatch and lower YFBM at hatch. The resolving effect of CO2 demonstrates that CO2 only seem to have a temporary effect during the hatching phase.