Glasses with compositions Li1.2M0.2Ge1.8(PO4)(3) (M = Al, Ga, Y, Gd, Dy, and La) were prepared and converted to glass-ceramics by heat treatment. The effects of the M3+ ions on the conductivity of the glasses and glass-ceramics were studied. The main phase present in the glass-ceramics was the conductive phase LiGe2(PO4)(3). Al3+ and Ga3+ ions entered the LiGe2(PO4)(3) structure by replacing Ge4+ ions, but lanthanide ions did not. The glass-ceramics exhibited much higher conductivity than the glasses. With increased ionic radius of the M3+ ions, the conductivity remained almost unchanged at similar to 3 x 10(-12) S/cm for the glasses, but it decreased from 1.5 x 10(-5) to 8 x 10(-9) S/cm for the glass-ceramics at room temperature. The higher conductivity for Al3+ and Ga3+-containing glass-ceramics was suggested to result from the substitutions of Al3+ and Ga3+ ions for Ge4+ ions in the Li(PO4)(3) structure.