This paper records the "Peixe-Rei" fishes from the Uruguayan fauna, giving a description of five new species, one genus and one sub-genus. The author reminds initially that for the time being no exaustive studies have been published concerning specimens of the family Atherinidae belonging to the fauna of Uruguay, Argentine and Brazil. The only works available on the subject are based on widely scattered and occasionally gathered material or on some other work not always founded on sound ichthyological methods and according to general scientific principles. This situation is the source of insurmountable difficulties in the comparative study and clear distinction of the various specific units that form the group. It is therefore the aim of this work to present a selective collection of the available literature concerning the "Peixe-Rei" of Uruguayan waters, making a comparative study of the preserved forms and proposing their classification according to the current systematic methods. The species under study being of economical importance, not only for food but also from the sportive point of view, special reference is made of artificial hatching, this section being already so important that a new branch of fish culture has been created, the Atherinoculture. A retrospective study is made of the authors that have investigated the existing genera in the South Atlantic and that are also found in inland waters of Uruguay and Argentine following with a special description of the Atherinoculture pointing out the example of Mexico that has obtained good results in its Limnologic Station at Patzcuaro. The author presents a description of the researches concerning Eslopsarum bartoni bartoni carried out at the Station. The Biometry of the "Peixe-Rei" is discussed in all its descriptive elements. Extensive considerations are made regarding Odontesthes followed by an analysis of the species included in this genus and a description of O. humensis nov. sp., O. guazu nov. sp. is given with a special reference to O. orientalis De Buen. The author creates the sub-genus Tupa and the genus Yaci, the first mentioned including O. platesis (Berg) and the second Y. retropinnis De Buen, nov. sp. Reference is made to the genus Kronia of Miranda Ribeiro (1915), pointing out that the several authors who have studied the validity of this genus overlooked a most important and distinctive character, i. e.: "ventral fins fused". A description of K. alba nov. sp. and K. rex nov. sp. is given. The writer presents his views on the genus Austroatherina Marrero and the species A. incisa (Jenyns) = Menidia uruguayensis Devincenzi. All analysis are accompanied by explanatory figures totalling 44 drawings and one photo.