The analysis of symbiotic systems' occurrence in the representatives of several protists' macrotaxa with various patterns of cellular organisation (amoebae, flagellates and ciliates) has been performed. The frequency of symbionts' occurrence in different cell compartments has been evaluated on the basis of data obtained from literature and original electron-microscopical investigations. Cell coat, cytoplasm, perinuclear space, karyoplasm and various cell organelles has been studied. No reliable cases of symbionts' maintenance in protists' mitochondria and plastids were established. The occurrence of symbionts' associations in different taxa varies essentially. Symbiotic systems of three protists' groups analysed are shown to possess qualitative differences. In amoebae (Lobosea, Gymnamoebia) in 24 species out of 61 species studied symbionts are found in cytoplasm only; they are maintained neither on the cell coat nor in the nucleus and other organelles. Symbiotic systems of flagellates and ciliates in general have much in common. For each of these groups more than a hundred species of symbionts are described. Symbionts occupy the cell surface, cytoplasm, karyoplasm and perinuclear space. For different ecological groups of flagellates high frequency of occurrence of various symbionts on the cell surface is noted (in more than 30 species), reports of ectobionts in ciliates being less numerous and mostly restricted to metanobacteria. The results of analysis performed show that the formation of symbiotic relationship in lower eukaryotes' evolution is not a partial result of accidental ancestral partners' combinations. It is a natural phenomenon, widely distributed of protists. It is pronounced in diverse forms and with various frequency in different evolutionary branches. We suppose that the regularities of appearance of stable symbiotic associations are determined by the pattern of cellular organisation of the host to a great extent and depend upon the morphophysiological preadaptations of prokaryotic microorganisms trying to use the protists' cells as their environment in the course of their co-evolutionary process.