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Parasites usually have strong negative effects on the fitness of their hosts, as in the reduction of host fecundity. We evaluated the female fecundity of Palaemonetes argentinus from two populations with contrasting levels of parasitism: the population from Nahuel Rucá (NR), Argentina that was fully parasitized by a larval digenean (Microphallus szidati) and the population from El Burro (EB) that was completely free of parasites. Realized and actual fecundity (i.e., number of newly spawned embryos and number of embryos ready to hatch, respectively) were higher in the parasitized females from NR. Moreover, infected females from NR produced heavier eggs than uninfected ones from EB. Egg loss (estimated as difference between realized and actual fecundity), however, was higher in the parasitized population (18.6 versus 10%). Higher egg loss is likely a negative effect of parasitism; however, the differences in …