Allorecognition is important for the development and social behaviour of Dictyostelium and helps the amoebae to distinguish kin from non-kin and preferentially cooperate with related strains. Allorecognition is mediated by two membrane proteins with extracellular immunoglobulin domains, TgrB1 and TgrC1. The transition from a unicellular to multicellular development requires interactions between matching types of TgrB1 and TgrC1, and earlier work from the groups of Gad Shaulsky and Adam Kuspa suggested that TgrB1 might function as a receptor. They therefore hypothesised that TgrB1 and TgrC1 act as receptor–ligand pair, and in this study (p. 4002) set out to test this theory by using cooperate aggregation and prespore differentiation assays with different types of TgrB1 and TgrC1. The results presented here confirm that TgrB1 indeed acts as a receptor and TgrC1 as its ligand. Interestingly, the authors further show that the cytoplasmic tail of TgrB1 is required for both cooperative aggregation and differentiation, as well as for mediating the signalling function. Indeed, deletion of the cytoplasmic tail of TgrB1 had nearly the same effects as those seen by deleting the entire receptor. Further evidence for a receptor function of TgrB1 comes from the observed phosphorylation of its cytoplasmic region upon binding of compatible TgrC1 in trans. Taken together, this work provides important new insights into the mechanisms of allorecognition that will serve as the basis to further explore the downstream signal transduction pathways.