An investigation on the development of P. catenata, Chlorella sp., S. bacillaris, and S. acutus in mixed cultures showed that in 7 out of 11 variants of these cultures the dominating species was that whose specific growth rate was higher in monoculture. This phenomenon was not observed in the bialgal culture of Chlorella sp. and S. bacillaris and in 3 mixed cultures which, apart from other algae, included P. catenata and S. acutus. In the latter P. catenata always dominated, its growth being much faster than in the monoculture.