Gigartinine, 5-(3-amidinoureido)-2-aminovaleric acid, and L-citrullinyl-L-arginine were islated from aqueous extracts of Chondrus crispus (Rhodophyceae). Their identifications were confirmed by chemical procedures, and 1H and 13C nuclear magnetic resonance and infrared spectroscopic methods. Citrullinylarginine, gigartinine, taurine, citrulline, and glutamic acid were the predominant free amino compounds. Citrullinylarginine showed the most pronounced change in concentration. This occurred during the winter months when it reached a maximum (in March) of 58 mumol/g fresh weight, a value 10 times greater than that of any of the free amino acids and equal to 50% of the total organic nitrogen in the plant; All of these compounds were depleted to a minimum of about 1 mumol/g fresh weight in October. Both gigartinine and citrullinylarginine were detected in Ahnfeltia plicata, Gracilaria sp. Petrocelis middendorfii, Polyides rotundus, Polysiphonia lanosa, and Rhodomela confervoides.