Abstract: In a tractor having a mid-PTO shaft (13) and a rear-PTO shaft (22) which are journalled in a transmission casing (10) disposed between left and right rear wheels (12), a PTO-selecting clutch (20) for selectively operating both of or any one of the PTO shafts is disposed on a rear end portion (13a) of the mid-PTO shaft. The selecting clutch includes an input gear (17), drivenly connected to a drive shaft (14) through a PTO-clutch (15) and through a first gearing (16), and an intermediate gear (19) drivingly connected to the rear-PTO shaft. In a preferred embodiment, a second rear-PTO shaft (24) is provided on which the intermediate gear is fixedly mounted. The selecting clutch disposed on the mid-PTO shaft contributes to make the structure and parts of power take-off transmission assembly largely common between tractors having both of the mid- and rear-PTO shafts and having only a mid-PTO shaft.