From February 1987 to February 1989, the populational biology of Contraceacum sp. (larvae) in its paratenic host, the fish Serrasalmus spilopleura Kner,1860, was studied in two ponds in a subtropical permanent habitat northeastern of Argentina. Fishes from Ramada Paso pond presented 80% of prevalence and 1 to 132 larvae per fish while fishes from Aeroclub pond presented 63% of prevalence and 1 to 184 larvae per fish. Fishes collected from Aeroclub pond have shown a high prevalence of infection during the first period of study (1987), diminishing the following year. In fishes from Ramada Paso pond the prevalence varied not significatively during the two years. Prevalence and mean intensity of infection increase with body length and weigth of the hosts. Sex of hosts is not an influential factor in parasitic level. The lenitic "closed" environmental (Ramada Paso pond) evidenced the greatest larvae mean intensity and prevalence. Althougt, the lenitic"open" environmental (Aeroclub pond) showed the greatest parasitic number of individuals in an infrapopulation. The spatial dispersion in both ponds were agregated and fit well a negative binomial model. Neverthelees, the Aeroclub pond presented the greastest overdispersion.