This paper presents data on the fish populations of temporary lenitic water bodies along the Middle Paraná River in the Santa Fe and Corrientes areas, some 650 km apart, sampled during 1972. Comparisons are made among the fish faunas present in the various ponds in 1972, and also with data collected in the same areas in the previous four years. Differences in population size, structure, specific diversity and dominance of particular species, are established between the populations of the two areas. The main reasons for differences are considered in relation to the hydrological regime.