A trocar assembly (70) formed from a trocar tube subassembly (74) and a separable, interfitting trocar subassembly (72). The trocar tube assembly includes a head (112) having a bore (120) into which a trocar tube (116) is fitted and a manually operated slide valve assembly (118) mounted transversely of the bore in the head. The valve has a bore (134) through it that may be aligned with the bore in the head by movement of the valve. The trocar subassembly includes a head (76) that has a bore (92) extending partly through it in which a trocar obturator (82) and a concentric tubular shield (86) are mounted such that the shield is capable of limited axial movement relative to the obturator between a normal, extended position in which the shield effectively covers the piercing tip (84) of the obturator and a retracted position in which the tip is exposed. The obturator and shield are inserted through the bore in the trocar tube subassembly head, the slide valve bore and the trocar tube. The bore in the slide valve has a radial tooth (136) in it that engages a slot (138) in the shield to reversibly lock the shield in its extended position when the slide valve is in its normal position. The valve is depressed manually to disengage the tooth to allow axial movement of the shield or separation of the trocar subassembly from the trocar tube subassembly.