Competition of invertebrates was studied in mixed culture (the ciliates Paramecium caudatum and Paramecium bursaria and the rotifer Brachionis plicatilis) under laboratory conditions. The invertebrates were fed on various food sources: 1) chlorella biomass and 2) biomasses of bacteria and yeast. The ciliate P.caudatum can feed on yeast, bacteria and chlorella. The P.bursaria – Chlorella condutrix endosymbiosis is a natural model of a simple biotic cycle. P.bursaria consumes glucose and oxygen released by zoochlorella in the process of biosynthesis and releases nitrogenous compounds and carbon dioxide necessary for algal photosynthesis. The rotifer Br. plicatilis can feed on algae, bacteria and detritus.      P.caudatum was found to be a poor competitor: even when the initial percentage of P.caudatum cells was large and there was no food limitation, this ciliate had been expelled by the end of the experiment. The main reason for the low competitive ability of P.caudatum in the closed system must be inhibiting effect of invertebrates’ metabolites, which has been confirmed by experimental results and literature data.       The outcome of the competition between the ciliate P. bursaria and the rotifer Br. plicatilis was different depending upon the food source: 1) chlorella or 2) bacteria and yeast. When fed on chlorella, P. bursaria was a better competitor than Br. plicatilis. With bacteria and yeast used as food, the opposite outcome of competition was observed: the competitive ability of Br. plicatilis was better than that of P. bursaria. It was assumed that due to turbidity of the culture solution in experiments with bacteria and yeast used as food, the zoochlorella photosynthesis rate decreased and, hence, the P. bursaria – zoochlorella symbiosis became less competitive.      Analysis of experimental results of growing P. bursaria and P. caudatum single species cultures in the light and in the dark, with bacteria used as a food source, confirmed that assumption. The results of the experiments confirmed the beneficial role of symbiotic relationships of the ciliate P. bursaria and zoochlorella. The number of symbiotic units in the light reached 220 units/ml, while in the dark it was much smaller. The P. caudatum number remained practically the same in the light and in the dark.