OKADA, Edson Kiyoshi; AGOSTINHO, Angelo Antonio; PETRERE JUNIOR, Miguel; PENCZAK, Tadeusz. Factors affecting fish diversity and abundance in drying ponds and lagoons in the upper Paraná River basin, Brazil. International Journal of Ecohydrology & Hydrobiology, Warsaw, v.3, no.1, p. 97-110, 2003.Monthly samples were taken from April 1992 to March 1993 of fish assemblages present in six permanent lagoons and four ponds, fragmented from them during the drought period and during the terminal phases of desiccation, to evaluate changes in their diversity, abundance and dominance. Their relationships with hydrological and environment conditions were examined. A total of 63 fish species were identified. Species composition and abiotic factors were heterogeneous among the different water bodies. The proportion of piscivores and oxygen concentration were the determining factors for structuring the assemblages. The impact of piscivory upon fish diversity was neutral at the beginning and negative at the end of the dry season. In the terminal phase of desiccation, diversity was higher in ponds that contained a moderate proportion of piscivores than in these containing a higher proportion.