Tritrichomonas foetus strongly agglutinates human erythrocytes, thus suggesting the occurrence of an adhesin associated with its surface. Adherence was observed immediately after mixing of the parasites with erythrocytes and the intensity increased for up to 30 min. Scanning electron microscopy examination of T. foetus-erythrocytes attachment showed that trichomonad cytoadherence took place mainly through their anterior and recurrent flagella. Ultrastructural observations showed that T. foetus contacts human red blood cells through punctual binding, inducing the separation between the two lipid monolayers of the parasite plasma membrane. This structural modification was also seen in freeze-fractnre replicas where protrusions on the P and depressions on the E fracture faces were observed. No intramembranous particles, which mainly correspond to membrane integral proteins, were observed at the adhesion areas, indicating lateral mobility of integral membrane components involved in the appearance of the intramembranous particles. However, no changes was observed on the surface coat.