Colonies of the ascidian Botryllus schlosseri may fuse with kin to form chimaeras which vary their life histories depending on environmental conditions. We placed chimaeric colonies of this species in Monterey Bay, California, U.S.A., where they received planktonic food continuously. In the field, chimaeras grew rapidly, attained large sizes, and produced many eggs. They formed compact disc-shaped colonies in which genotypic composition remained stable throughout their lifespan. In most cases, genotypic partners in field chimaeras senesced and died synchronously. We also cultured genetically identical replicates of the same chimaeras under laboratory conditions, where they were fed once daily. In the laboratory environment, chimaeras grew slowly, shrank, and fragmented. Most genotypes in chimaeric colonies produced significantly fewer zooids and eggs in the laboratory than they did in the field. Somatic cell parastitism, in the form of resorption of tissues of one genotype by the other, occurred mainly in the laboratory environment, and not in the field. The phenomenon of resorption may thus be a dispensible strategy of fused genotypes depending on environmental conditions. Genotypes in field chimaeras may grow and reproduce rapidly because of the non-limiting food resources available. These data demonstrate that chimaeras of B. schlosseri have extremely plastic life histories, and employ different strategies depending on the environment.