DEITOS, Claudenice; AGOSTINHO, Angelo Antonio; SUZUKI, Harumi Irene. Population structure and reproductive biology of Loricariichthys platymetopon (Siluriformes, Pisces) in the upper River Paraná = Estrutura populacional e biologia reprodutiva de Loricariichthys platymetopon (Siluriformes, Pisces) no alto rio Paraná. Brazilian Archives of Biology and Technology = (Arquivos de Biologia e Tecnologia), Curitiba, v.40, no.4, p.793-807, 1997.Distribution, length, and reproductive biology of Loricariichthys platymetopon were analyzed in different environment of the upper Paraná River. Samples were taken monthly from October 1986 to September 1988 using gillnets with different mesh size, being caught 7691 individuals. This species was more constant and showed higher densities in lentic environment than in other ones. Reproductive activity was more intensive in lentic environment, and under condition of low transparency and low concentration of dissolved oxygen. Spawning was observed from September 1986 to January 1987 when flood was absent and from September 1987 to April 1988 under normal flood conditions. Transportation of the eggs (340 on average) under the lips by males is a strategy appropriate for varying water levels and the limnological factors of floodplain lagoons, the main spawning environment.