Several lines of evidence from a study of razorbill mating strategies indicate that females determine whether extra-pair copulations result in insemination: a. Direct observation suggested that males could not overcome female resistance; b. Females allowed repeated EPC attempts from the same male when they could have departed; c. No EPC attempts of post-egg-laying females were successful, but many were successful before laying, suggesting that females permitted insemination only when they could benefit from it; d. Males sometimes ceased attempting EPCs with females who persistently demonstrated unwillingness to copulate; e. Female behaviour has raised the possibility that sperm may be expelled. Two additional lines of evidence are deduced from a comparison of razorbill data with published accounts of the closely related common guillemot Uria aalge, in which forced EPCs are reported to occur regularly; (f) Male guillemots guard their mates with much greater vigilance than male razorbills, and (g) Male guillemots attempt EPCs by disrupting within-pair copulations and trying to force the pair female, while male razorbills decline such opportunities. The ability of females to control EPCs is important because it enables them to pursue a mixed reproductive strategy without suffering the costs of unwanted inseminations.