In the wild, social groups of the cellular slime mould amoeba Dictyostelium giganteum are genetically heterogeneous more often than not. When studied as 1:1 binary mixes, amoebae of one strain almost always form more spores than the other, an observation that leads one to wonder what might be responsible for the long-term persistence of different strains in nature. We have monitored a number of individual and collective traits bearing on reproductive fitness in chimaeras of Dictyostelium giganteum obtained by mixing pairs of starved amoebae belonging to distinct wild-type strains in proportions ranging from 1:9 to 9:1. The main findings are that intercellular interactions take place at more than one stage of the life cycle and that generalisations drawn after mixing cells only in a 1:1 ratio can be misleading. A strain that does better than another in respect of some component of fitness (for example, spore formation) may do worse in respect of a different component (for example, growth rate). Also, a strain that is a more efficient sporulator than a second strain in one context may be less efficient in another context. In addition to such trade-offs, spore formation in chimaeras can exhibit negative frequency dependence, which too can lead to stable co-existence.