Studies on infection patterns of diplostomid parasites in commercially exploited fishes have not been done in Patagonia (Argentina). The aim of this work was to study the population dynamics of two diplostomid species in the brain of Patagonian silversides (Odontesthes hatchery), the interaction between them, and effect on health and physical condition of the hosts. Tylodelphys destructor and Diplostomum mordax metacercariae in the brain of Patagonian silversides in Lake Pellegrini were studied between January 1991 and February 1992. Tylodelphys destructor parasitized all silversides examined; prevalence of D. mordax varied between 7% and 100%. Mean intensity for T. destructor was 35–140 and for D. mordax was 3–49. Highest mean intensities of T. destructor coincided with the lowest mean intensities of D. mordax. Recruitment seems to occur from July–November for T. destructor and from April–June for D. mordax, revealing a temporal segregation with inverse patterns of infection and recruitment. Tylodelphys destructor has higher intensities in the brain of the older fish, whereas D. mordax did not, suggesting another type of segregation. There were no evidences of gross pathology. No covariation between abundance of larvae and condition factor, gonadosomatic index, and gut fullness was detected.