The glass‐forming region was determined for the system Ge‐Bi‐Se; 13 at.% Bi was found to be incorporated at its maximum content into glasses, the Ge content of which was 20 at.%. Electrical resistivity, thermoelectric power, and optical absorption coefficient were measured mainly on a series of Ge20BixSe80−xglasses (x=0 –13). The resistivity decreased by about four orders of magnitude betweenx=9 and 10, and remained almost constant forx?10. The thermoelectric measurement showed the change of conduction type frompton, accompanied by the above‐mentioned abrupt decrease of resistivity. Inn‐type glasses, hopping conduction in the tail of localized states was proposed in parallel with conduction in extended states. The optical band gap was very slightly changed with the incorporation of more than 2.5 at.% Bi, in contrast to the remarkable decrease in the activation energy for conduction betweenx=9 and 10. The discussion based on the concentration of covalent bonds formed in the glasses led to the conclusion that the formation of a fairly large number of Bi‐Se bonds and the disappearance of Se‐Se bonds nearx=10 were closely related to the composition dependence of the electrical and optical properties of the glasses in the present system.