Interactions between the blue-green alga Pseudoanabaena catenata and the green alga Stichococcus bacillaris were investigated in bialgal cultures carried out in a chemostat, on a solid medium, and in continuous cultures with rotating disks. The inhibitory effect of S. bacillaris on P. catenata was most pronounced in the continuous cultures carried out with liquid media of slightly acidic pH and in the culture with rotating disks in S. bacillaris filtrate. The growth of P. catenata was also strongly inhibited in cultures on the solidified medium. The least inhibition was observed when continuous cultures were carried out in the chemostat in a slightly alkaline medium.