A belt drive transmission comprises a plurality of belt pulleys over which passes at least one stretched drive belt composed of elastomeric material and having a substantially rectangular cross-section. At least one of the pulleys, especially a relatively small diameter drive pulley, has a belt-receiving peripheral groove of which the main side portions lie at different angles to a plane perpendicular to the axis of the pulley. The bottom of the groove is formed with a radially inwards convergent channel of which the width at the widest part is substantially equal to the width of the narrower edge of the belt while the depth of said channel is at least as great as the width of the narrower edge of the belt. In a stationary non-driving condition the belt lies against the main side portion of the groove which is at the greater angle to said plane perpendicular to the pulley axis, but under driving conditions at speed the belt moves into said channel.