Both deleterious mutations and parasites have been acknowledged as potential selective forces responsible for the evolutionary maintenance of sexual reproduction. The pluralist approach to sex proposes that these two factors may have to interact synergistically in order to stabilize sex, and one of the simplest ways this could occur is if parasites are capable of causing synergistic epistasis between mutations in their hosts. However, the effects of both deleterious mutations and parasitism are known to be influenced by a range of environmental factors, so the nature of the interaction may depend upon the organisms’ environment. Using chemically mutated Daphnia magna lines, we examined the effects of mutation and parasitism under a range of temperature and food regimes. We found that although parasites were capable of causing synergistic epistasis between mutations in their hosts, these effects were dependent upon an interaction between parasite genotype and temperature.