Active uptake of phenylalanine has been characterized in a species of cyanobacterium, Anabaena sp. ATCC 27893. Phenylalanine was apparently taken up by only one transport system over the concentration range 2–50 μM. Inhibition of phenylalanine uptake by glutamate and a range of neutral amino acids suggested that these were internalized by the same system. At least one other system appeared to be present, transporting the basic amino acids arginine and lysine. The Km and Vmax of phenylalanine uptake were comparable to those of phenylalanine uptake in eukaryotic unicellular freshwater algae. Uptake was energy dependent and internalized phenylalanine was rapidly incorporated into trichloracetic acid-insoluble material.