Gisela pela orientação, confiança, amizade e paciência. À Mônica pela amizade, pelo incentivo e a sua disposição e paciência em compartilhar todo seu conhecimento, e pela ajuda valiosa nas identificações. Ao Hélio pela amizade e por agüentar muitas vezes meu "estresse". Ao auxílio nas triagens dos Estagiários, Fernanda, Melissa, Fabi, Júlia, Eduardo, das Bolsistas Gláucia, Josi, Vanessa, Mirela e ao Técnico de Laboratório Emerson. Ao pessoal do Setor de amostragens da CETESB, pela amizade e sempre valiosa colaboração nos trabalhos de campo. As chefias da Divisão de Análises Hidrobiológicas, Dra. Marta Conde Lamparelli e do Setor de Comunidades Aquáticas MSc Guiomar Johnscher-Fornasaro, da CETESB, pelo incentivo, apoio e disponibilização dos dados sem os quais não seria possível a realização desta tese. Ao Prof. Dr. Roberto Shimizu pela disposição e esclarecimentos de dúvidas. Ao Roque pela amizade, incentivo, paciência em auxiliar na parte estatística e leitura e sugestões dos capítulos da tese. Aos amigos Rosalina,

Aim The order Ephemeroptera, together with Trichoptera and Plecoptera, has been considered as sensitive in environmental quality assessment. However, the species Campsurus truncatus Ulmer 1920 (Polymitarcyidae) has been sampled in anthropically altered sites. In this paper, data on the occurrence of that species in the water quality monitoring network of the Environmental Agency of São Paulo State (CETESB) were gathered to review its position in the environmental gradient and in the quality assessment of rivers and reservoirs. Methods For this, a compilation of the occurrences and densities of Campsurus obtained in rivers and reservoirs of São Paulo state between 2001 and 2018, with grab samplers, was carried. Environmental factors, the Trophic State Index (TSI) and the Benthic Community Index (BCI) were related to the species densities to define its environmental requirements. Results The sublittoral zone of reservoirs was the most common habitat to C. truncatus, occurring more frequently at mesotrophic sites with fair ecological quality. Higher mean densities were recorded at eutrophic sites with good ecological quality conditions. The statistical analysis did not show linear correlation between C. truncatus densities and any of the analyzed environmental variable such as grain size, organic matter content, DO and light penetration, although there was a tendency for higher species densities occur in muddy, dark sediments and with intermediate levels of organic matter. As described in literature the presence of aquatic plants can actually benefit that population once the highest densities were observed in sites with many macrophytes, as Salto Grande reservoir, but it may be confusing the relationship between population density and environmental factors. Conclusions The high occurrence and densities of C. truncatus obtained in eutrophic environments suggest that the species should not be considered as a sensitive taxon in qualitative indexes, such as the BCI for rivers and the sublittoral region of reservoirs. Moreover, its exclusion from the richness of sensitive groups had low impact on the diagnoses already performed at the CETESB monitoring network.