It is suspected that phagotrophic marine protozoa might possess feeding receptors that enable them to discern the nutritional quality of individual prey items (during prey-handling) on the basis of their cell-surface biochemistry. This article reviews advances in our understanding of the molecular mechanisms that mediate the biorecognition and selection of nonself (microalgal) prey items by the microplanktonic marine phagotroph Oxyrrhis marina. The potential importance of lectin-glycan interactions is first considered in view of findings which demonstrate that O. marina possesses lectin-like feeding receptors specific for prey-surface (mannose) glycoconjugates. Secondly, some conceptual bases for indirect or 'opsonic' modes of prey biorecognition mediated by soluble prey-labelling proteins are presented. Finally, the possibility that some accounts of selective feeding in O. marina might result from the noxious effects of prey-associated chemicals rather than active 'distaste' by phagotrophic cells is discussed. Recent evidence for toxic superoxide (O(2)(-)) production by marine microalgae is afforded particular attention given that release of O(2)(-) anions can be exacerbated by the binding of mannose-specific lectins to the microalgal cell wall; a novel model for grazing-activated chemical defence is proposed.