Resumo. Abstract Aim To evaluate the contribution of naturalists Francis de Castelnau, Jacob Heckel, Johan Baptist von Spix, Johann Natterer, Louis Agassiz, and Rudolf Kner to the knowledge of taxonomic diversity of freshwater ichthyofauna in Brazil between the years 1829 and 1859. Methods Two data matrices were constructed: one with information on freshwater fish species known in Brazil until 2021 and another based on the list of fish species in South America presented by Castelnau (1855). Both were supplemented with information on taxonomy, geographic distribution, authorship, and geographic distribution. The Taxonomic Diversity Index (Δ) was calculated from the first matrix to assess the contribution of the naturalists and determine the number of species per hydrographic region. The second matrix allowed for a detailed analysis of Francis de Castelnau's contribution. Results Between 1829 and 1859, the five naturalists considered in this study described 171 fish species in Brazil, including several endemic species. Castelnau was the naturalist with the greatest contribution (Δ=87.7), followed by Agassiz (85.5), Spix & Agassiz (85.4), Kner (80.8), and Heckel (48.7). The five naturalists described species from four hydrographic regions: the Amazon (122 species), Paraná (25), Tocantins-Araguaia (18), and São Francisco (13). The 276 fish species cataloged by Francis de Castelnau are mostly from the Neotropical region, including some endemics, originating from watercourses in the southeastern Brazilian region, encompassing species collected for the first time in the watercourses of Central Brazil and some marine species, ranging from small (5 cm) to medium (56.0 cm) in size. Conclusions The five naturalists considered in the study made a significant contribution to the early scientific knowledge (1829-1859) of the Brazilian ichthyofauna, but this contribution varies among the naturalists, with particular emphasis on Castelnau, Agassiz, and Spix & Agassiz, as well as among the hydrographic basins, notably the Amazon.