Prezygotic reproductive isolation has traditionally been studied from a precopulatory perspective even though postcopulatory events have been known to influence success at fertilization. Postcopulatory, prezygotic reproductive isolation (gametic isolation) has received relatively little attention mainly because the focus of research has been on the events that occur either earlier (pre-mating) or later (postzygotic) in the chain of events that lead to successful reproduction. However, recent evidence from an array of taxa from sea urchins to beetles reveals that postejaculatory, prezygotic events may be an important factor in reproductive isolation. Such a mechanism may be the driving force behind speciation in some taxa, such as free-spawning marine invertebrates, whilst it may complement pre-mating and postzygotic mechanisms in others. In this paper we draw an analogy between sperm–egg interactions and classical sexual selection theory and argue that gametic incompatibility may arise through cryptic female choice.