Resumo. Abstract Aim: To analyze temporal distribution of larvae and juveniles and the early development and of Moenkhausia cf. gracilima. Methods Samples were taken quarterly in twenty-five sites in the upper Paraná River floodplain between August 2013 and May 2015. The samples were taken under the water surface at night using 0.5 mm mesh plankton nets. In the laboratory, samples were sorted, identified and separated into larval (preflexion, flexion and postflexion) and juvenile periods. Results A total of 248 individuals was collected, mainly in the Saraiva Lagoon, suggesting that the entire life cycle of this species occurs in this environment. The reproductive period takes place between December and April, since the postflexion larvae were found until May. However, the occurrence of juveniles between February and May indicates probable batch spawning. Among the 95 individuals used for ontogenic description, 82 were larvae and 13 juveniles. Larvae may be characterized by irregular pigmentation in the upper region of the head, mouth, and body, increasing throughout development; upper lobe of the caudal fin more pigmented than the lower lobe, only visible in postflexion larvae; terminal mouth; anal opening located anterior to the median region of the body and total number of myomers ranging from 34 to 40 (15 to 20 pre and 16 to 23 postanal), while juveniles have characteristics similar to adults. The total number of fin rays is: P. 11-16, V. 7-11, D, 9-11 and A. 21-23. Conclusions According to the distribution of developmental periods it is possible to conclude that this species reproduces in the summer, preferably in lagoons. Growth analysis indicated important alterations in larval morphology (metamorphosis) that may be associated with the ecomorphological characteristics of the species. The morphological separation of larvae of M. cf. gracilima from other larvae of small characids, especially at preflexion and flexion stages may be complicated by the overlap of traits, suggesting the use of other variables, mainly morphometric, for the separation of the species.