Background: A major challenge for evolutionary biology is explaining altruism, particularly when it involves death of one party and occurs across species. Chimeric fruiting bodies of Dictyostelium discoideum and Dictyostelium purpureum develop from formerly independent amoebae, and some die to help others. Here we examine co-aggregation between D. discoideum and D. purpureum, determine its frequency and which party benefits, and the extent of fair play in contribution to the altruistic caste.Results: We mixed cells from both species in equal proportions, and then we analyzed 198 individual fruiting bodies, which always had either a D. discoideum or D. purpureum phenotype (D. discoideum-98, D. purpureum-100). Fifty percent of the fruiting bodies that looked like D. discoideum and 22% of the fruiting bodies that looked like D. purpureum were chimeric, though the majority of spores in any given fruiting body belonged to one species (D. discoideum fruiting bodies-0.85 +/- 0.03, D. purpureum fruiting bodies-0.94 +/- 0.02). Clearly, there is species level recognition occurring that keeps the cells mostly separate. The number of fruiting bodies produced with the D. discoideum phenotype increased from 225 +/- 32 fruiting bodies when D. discoideum was alone to 486 +/- 61 in the mix treatments. However, the number of D. discoideum spores decreased, although not significantly, from 2.75e(7) +/- 1.29e(7) spores in the controls to 2.06e(7) +/- 8.33e(6) spores in the mix treatments. D. purpureum fruiting body and spore production decreased from 719 +/- 111 fruiting bodies and 5.81e(7) +/- 1.26e(7) spores in the controls to 394 +/- 111 fruiting bodies and 9.75e(6) +/- 2.25e(6) spores in the mix treatments.Conclusion: Both species appear to favor clonality but can cooperate with each other to produce fruiting bodies. Cooperating amoebae are able to make larger fruiting bodies, which are advantageous for migration and dispersal, but both species here suffer a cost in producing fewer spores per fruiting body.