Summary: The virus of fowl pox has been grown in roller tube cultures of chick fibroblasts in which it survived and multiplied through constant association in low titers with the same continuous cultures for a period of over a hundred days. The cells failed to show any inclusion or destruction. Epithelial cells in culture are killed by the virus, but without the formation of definite inclusion bodies. The refractile inclusion body of the chick chorio-allantoic membrane is osmophilic and can be partially extracted with ether. The residual inclusion is still resistant to purified desoxyribonuclease, ribonuclease and trypsin. The inclusion can be dissolved and elementary bodies liberated by treatment with sodium lauryl sulphate (Duponal C) but this in the concentrations used inactivates the virus. The individual elementary bodies as seen in the electron microscope have a central dark area and are brick shaped. This is brought out more clearly by acid peptic digestion. Smear preparations of the whole inclusion show an amorphous material, presumably fat, surrounding the individual bodies. Some inclusions have thin threads of material running through this matrix. Thin sections of infected membranes show an intracellular edema at the start of the infection, then swelling and destruction of the mitochondria. Few virus particles are present at the start. The number of virus particles and the amount of fat increase to form the final dense osmophilic inclusion.