A trocar assembly consisting of a trocar tube subassembly and a separable, interfitting trocar subassembly is described. The trocar tube assembly includes a hollow body having a first front opening in which a trocar tube is mounted, a second rear opening for receiving the trocar subassembly, and a manually pivotable flap valve mechanism mounted within the body. The trocar subassembly includes a head having a central bore in which are mounted the (1) rear ends of an elongated obturator having a pyramidal-shaped piercing head with a triangular-shaped base, and a concentric tubular shield having a triangular shaped front opening for the piercing head that is formed by bevels in its leading edge that have approximately the same slope as the surfaces that define the three sides of the piercing head, and (2) a coil spring backing the shield for biasing the shield between retracted and extended positions. When the two subassemblies are fit together, the obturator and concentric shield extend through the hollow body with the front wall of the head fitting within a recess in the rear wall of the hollow body such that the trocar assembly cannot be rotated without disengaging the two subassemblies. The shield has a slot in its outer wall that is engaged by the flap valve to lock the shield in its extended position. The shield may be unlocked by pivoting the flap valve out of the slot, sliding the trocar assembly rearwardly so that the front wall of the head is clear of the recess in the rear wall of the hollow body, rotating the trocar subassembly to a position in which the flap valve and slot are not aligned and sliding the front wall of the trocar assembly back into the recess in the rear wall of the hollow body.