This present work is a complete transcription of a talk given by Dr. Harald Sioli (1910-2004) at the Seminária da Amazönia, on 10 August 1990. The occasion was his last trip to Amazonas, when his collogues paid him homage on completing 80 years of age. On that occasion, INPA invited him to speak about the "50 years of limnology research in Amazonia", which, according to his own words, were "the most decisive days of his life". The text can be divided into two parts: the first refers to his personal history, namely, from his arrival in Brazil, his discovery of Amazonia and his survival in the backwoods of Amazonas and Pará to the 2nd World War, until he became a researcher at INPA and setting up the INPA/Max Planck working agreement; the second speaks of his limnology research, interacting with research, with Brazilian and foreign researchers of the past and present, and with backwoodsmen (caboclos) who, in his mind, "live with the forest and not against the forest". The transcription was made by Elci Silva and Antonio Alvarez, and kept by Terezinha Soares of the Assessoria de Tecnologia Extensão - ASTE, which, at the time, organized the Seminária da Amazônia. Dr. Sioli personally authorized the publication in 2000 by correspondence in which he declares: "I would satisfy me very much, if I could still be useful in the fight for the survival of Amazonia which is my second country". Now, at last, it is being published. Although Harald Sioli is no longer with us, this publication is homage of the Acta Amazonica to the memory of one who dedicated so many years to the knowledge and preservation of Amazonia.
Amazonia; limnology; equatorial climate; chemism