Acanthamoeba castellanii was isolated from soil and water as a predator of cyanobacteria. Several strains of A. castellanii consumed cyanobacteria but there were differences in predatory activity between strains. The unicellular Gloeocapsa alpicola and filamentous A nabaena 80s-aquae were particularly susceptible to predation. Under optimal conditions the amoebae fed voraciously but they encysted when the cyanobacterial food supply was exhausted. Predation was initiated by engulfment of whole cyanobacteria and, in the case of Anabaenacflos-aquae, severance of filaments. This was followed by digestion in vacuoles.