To investigate the effect of aflatoxin $B_1$, on survival rate and ultrastructure of liver during chick embryogenesis electron microscopic methods were used. After injection of aflatoxin $B_1$ into the yolk, ultrastructural changes in the liver of chicken embryo were observed. The results were as followed. 1. 12-day old chicken embryos were treated with single injection of aflatoxin $B_1$ with the dose of $0.0005{mu}g,;0.005{mu}g,;0.05{mu}g,;0.5{mu}g,;2.5{mu}g,;5.0{mu}g$ each. Chicken embryos treated with the dose of $0.5{mu}g$ of aflatoxin $B_1$ had survival rate of 22%. The embryos treated with $2.5{mu}g$ of aflatoxin $B_1$ hardly survived. 2. Chicken embryos treated with $0.05{mu}g$ of afatoxin $B_1$ had hatched in 30%, but once hatched, they all survived. 3. After administration of $0.05{mu}g$ of aflatoxin $B_1$ into the 12-day old chicken embryo, the electron microscopic studies were examined during development stages. The nuclei of hapatocytes became irregularly shaped and the structures of endoplasmic reticulum were changed to spherical types at 20-day old chicken embryo. Also, mitochondria became to be dilated and severe fibrosis was induced in the cytoplasm. However, the hepatocytes became almost normal in 30-day old young chicken.