Abstract: An interlock mechanism is provided for a transmission system having a gear unit with a plurality of selectable gear ratios. A manually operated clutch control pedal is manually movable from a clutch engaged position to a clutch disengaged position. The transmission has a pair of shift rails movable to select gear ratios. The shift rails have detent grooves which interact with a pair of detent balls. An interlock member is normally in a lock position wherein the interlock member engages both detent balls and holds them in the detent grooves to prevent movement of either or both shift rails. The interlock member is movable to a release position wherein the interlock member is disengaged from the both detent balls whereby the detent balls will allow one of the shift rails to be moved. An interlock spring is biased to urge the interlock member to the lock position.