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In a recent paper, Birkhead (1998) considers the criteria necessary to demonstrate that females are able to coordinate their use of sperm to bias paternity in favor of some males over others. This was a timely and much needed exercise, given the recent development of cryptic female choice theory (Eberhard 1996) and proliferation of interest in postcopulatory sexual selection in general. However, we disagree with Birkhead regarding the methods of demonstrating sperm choice. In this paper, we analyze Birkhead's (1998) criteria and indicate why they are collectively neither sufficient nor necessary to demonstrate female sperm choice. We suggest that statistical inferences of sperm choice require a pattern of variation different from that suggested by Birkhead. Furthermore, no matter how clear the patterns of variation, direct evidence of sperm choice will ultimately require either experimental manipulation of the …