Monthly data collected during 1993 in the Vaccarès lagoon (Rhône Delta, France) were used to investigate the influence of the trematode Aphalloïdes cœlomicola on the survival of the common goby Pomatoschistus microps, which acts both as second intermediate and definitive host. Prevalence of the parasite did not increase with fish size in either sex. Mean abundance and variance to mean abundance ratio gave evidence that the trematode could have an impact on host mortality. Gonad weight of parasitized females was reduced and the parasite seemed to provoke a gonadal regression above a certain parasitic biomass. Fecundity and egg diameters did not differ between parasitized and unparasitized females, we thus hypothesize that the energy content of eggs (composition in lipids and proteins) was probably modified as an adaptation to energy losses. Prevalence of infection in males compared to females and influence of the parasite on host survival and reproductive effort are discussed.