A unique aspect of phagocytosis during the sexual cycle of Dictyostelium discoideum is the ability of the zygote giant cell (ZGC) to attract and engulf hundreds of amoebae of the same species. The work presented here is one of two initial attempts to understand the signal transduction pathways that are involved during this event. Our data indicate that the uptake of amoebae by the ZGCs is negatively modulated by 5'-AMP and adenosine (ADO). The hierarchical inhibition of phagocytosis by the ADO analogues, N-ethylcarboxyadiadenosine and phenyl isopropyl adenosine, argue that the phagocytosis of amoebae by ZGCs is mediated by an ADO receptor, which is similar to the purinogenic class of receptor, A2. As with the A2 receptor, stimulation of cells by ADO binding causes an up-regulation of cAMP synthesis thereby increasing the intracellular levels of cAMP. During the phagocytic phase of sexual development, 5'-AMP and ADO undergo marked and successive increases in amounts supporting their natural role in modulating phagocytosis. This negative modulation of phagocytosis by ADO is similar to that found in mammalian macrophages and may represent an evolutionary precursor to that regulatory process. Furthermore, the slowing of phagocytosis has important implications to sexual development of D. discoideum.