Some characteristics of two viruses of the Douglas-fir tussock moth are described. Viral development appears to start in the nuclei of infected cells with the formation of tubes believed to be the capsids. Subsequently, the tubes are filled with viral nucleoprotein. Filaments which may be molecules of DNA are illustrated. Objects believed to be defective virus particles, consisting of capsids containing a subnormal quantity of nucleoprotein, are described. The bearing of these observations on the existence of an “intimate membrane” is discussed.