Do parasites and accumulation of deleterious mutations act synergistically in balancing the costs of sex? We addressed this possibility in the freshwater planarian flatworm Schmidtea polychroa. Sexual and parthenogenetic forms of this species sometimes coexist but show no ecological separation. Previous studies indicate that in a mixed sexual/ parthenogenetic population in Lago di Caldonazzo (N. Italy) parthenogens get more frequently infected with parasites. At the same time, they suffer from higher embryo mortality, which has been interpreted as a sign of accumulation of deleterious mutations. In the present study, we test whether these two factors are correlated, by focusing on the differences among the clonal lineages of a predominantly parthenogenetic subpopulation. Our results suggest that, for two out of three parasite types found, the infections are positively associated with the indirect measure of host mutation load.