Sexual cannibalism was examined experimentally in the New Zealand paddle crab Ovalipes catharus, where it is the female that risks cannibalism during and after sexual activity. In this species copulation only occurs when the female is soft-shelled after moulting and she is most vulnerable to cannibalism. Male paddle crabs protect and copulate with the females until the females are no longer sexually receptive. Males appear able to identify a female with whom they have recently copulated. After a brief separation, significantly fewer females were cannibalized in cases where a female was returned to her original sexual partner than in those that had exchanged partners. The asymmetry, where females gain some protection during moulting but males receive no such advantage, may, at least partially, explain the skewed sex ratios which are sometimes found in samples of O. catharus from nature.