AbstractThe subclass Peniculia (Oligohymenophorea, Intramacronucleata) is one of the most known groups of the phylum Ciliophora, being composed by very notorious representatives, such as Paramecium and Frontonia. Nevertheless, phylogenetic relationships among genera within this subclass are still far from being resolved. Moreover, for several members of the group the characterization by molecular markers is still lacking, such as for Wenrichia, Clathrostoma, Paraclathrostoma, Didieria and, to date, also for Neobursaridium. The finding of one strain of Neobursaridium gigas from India led to the first molecular characterization of this uncommonly sampled ciliate. The 18S rDNA sequence and the COI sequence were obtained and used for phylogenetic analyses. Moreover, the partial mitochondrial genome of N. gigas was sequenced, annotated and employed for phylogenomics analysis. To increase the sampling effort for the Paramecium clade, several newly obtained 18S rDNA sequences of parameciids are herein presented. Unexpectedly, the inclusion of N. gigas's molecular data in phylogenetics/phylogenomics analyses did not help to solve the complex evolution relationships inside Peniculia. Conversely, it raised new and intriguing questions about Paramecium phylogeny, since N. gigas clustered inside Paramecium clade as sister species of Paramecium bursaria in all the performed analyses. A critical revision of past and present data led to rename N. gigas as Paramecium gigas (Balech, 1941) comb. nov., and triggered the revision of genus Paramecium, with the proposal of the new subgenus Gigaparamecium subgen. nov. Hypotheses on the evolution of giant morphologies in ciliates are also discussed.