A trocar assembly comprising a cannula and an obturator comprising: a shaft 2 extending through the cannula and a head 4 arranged to protrude beyond the cannula where at least a portion of the head comprises an external diameter greater than the cannula internal diameter. The head has a radially extending cut-out 18 such that the head portion is radially collapsible for withdrawal through the cannula. The obturator has a locking member 30 which is received within the cut-out preventing withdrawal of the obturator. The head portion whose external diameter may be substantially equal to the cannula internal diameter may also comprise a shoulder 14 which engages the cannula front end. The head may have a tapering profile. The locking member may slid back axially to unlock the obturator. The obturator and locking member may comprise co-operating interlocking features. The locking member may comprise: an elongated rod extending through the obturator and at least one radially extending projection 34 engaging with at least a portion of the cut-out. Each projection may be received in a slot in the obturator. The cut-out may comprise one or more radial slots extending axially along both the head and the shaft of the obturator.
