The costae are cytoskeletal structures found in Trichomonadidae. Both the structural organization and composition of this organelle are still unknown. In the present work we have introduced a new methodology for the costa isolation. Using sucrose density-gradient centrifugation an enriched costa fraction was obtained. Analyses by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) showed that the costa contains several proteins, with major bands corresponding to apparent molecular masses of 122, 115, 112, 93, 87, 82, 59, 52, 44, 41, 32, and 26 kDa. No significant amount of carbohydrates was detected in the costa fraction. The fractionation methodology described here has the advantage of using normal centrifugation methods and is being applied to trichomonas in the size range of Tritrichomonas foetus and Trichomonas vaginalis.