The costa is an intracellular organelle common to all trichomonads. Costae from Tritrichomonas foetus have been purified by a method which involves lysis of T. foetus with the heat-stable hemolysin produced by Pseudomonas aeruginosa, followed by differential centrifugation. Analysis of the purified costae demonstrated that the organelle is composed of 95 percent carbohydrate and 5 percent protein. The carbohydrate moiety, probably a polysaccharide, consisted of glucose (95 percent), mannose (0.4 percent), glucosamine (1.4 percent), ribose (0.6 percent), and an unidentified sugar (2.6 percent). The kinetosomal complex was attached to the costa after initial lysis of cells but was separated from the costa during purification.