Abstract Of the 20,000 species of Annelida, most live in salt water, and the minority inhabits freshwater or terrestrial environments. Among the freshwater representatives, the family Aeolosomatidae draws attention mainly due to its enigmatic phylogenetic position. In the 1940s, Prof. Ernst Marcus was a pioneer in the study of Aeolosomatidae, mainly from collections in the state of São Paulo, Brazil. His research resulted in the description of eight species, four of which were new to science. Later, Marcus’ description of Aeolosoma headleyi was recognized as a fifth new species by Van der Land ( 1971 ) and named Aeolosoma marcusi in his honor. Recently, during a visit to the Museum of Zoology of the University of São Paulo (MZUSP), we found the original material deposited by Marcus ( 1944 ), but not formally registered. Thus, the aim of this work was to redescribe and designate the lectotypes and paralectotypes of Aeolosoma gertae Marcus, 1944 and Aeolosoma sawayai Marcus, 1944. We were able to study and photograph the specimens using a light microscope with Differential Interference Contrast (DIC). We were also able to identify and redescribe many important taxonomic features. However, some structures can only be observed in living individuals (as they are not preserved in the fixed material), which reinforces the need to study live specimens. This paper contributes to understand the diversity of Aeolosoma , provides new morphological knowledge and lays the foundation for new approaches to the study of aeolosomatids. 20000 20 000 20,00 Annelida water environments representatives position 1940s s Prof Brazil science Later 197 honor Recently MZUSP, MZUSP (MZUSP) 194 registered Thus DIC. DIC . (DIC) features However material, material) aeolosomatids 2000 2 00 20,0 19 (MZUSP (DIC 200 0 20, 1