Tritrichomonas foetus and Trichomonas vaginalis are protozoan parasites that cause sexually transmitted diseases in cattle and humans, respectively. There is a need for new antimicrobial agents to treat or prevent trichomoniasis because there are currently no approved chemotherapeutic agents against T. foetus and resistance of T. vaginalis to metronidazole does occur. Therefore, we evaluated the effect of a novel antimicrobial peptide, d-hecate, on the viability of 6 isolates of T. foetus and T. vaginalis in vitro. Tritrichomonas foetus and T. vaginalis were grown to mid log phase (24 hr) or late log/stationary phase (48 hr). Parasites at 10⁶/ml were mixed with equal volumes of d-hecate to final concentrations of 10 µM, 20 µM, and 40 µM of d-hecate. Controls had minimal essential medium (MEM) alone. The numbers of viable parasites were determined microscopically after 10, 20, and 30 min of incubation at 37 C with d-hecate or MEM. Our results show that d-hecate killed all 6 isolates of T. foetus and T. vaginalis evaluated. The killing effect was dependent on the concentration of the peptide, incubation time, and phase of growth of the parasites. Ultrastructural studies of parasites treated with 10 µM of d-hecate revealed extensive damage to the plasma membrane of most T. foetus and T. vaginalis cells, while a few cells were distorted but remained intact. d-Hecate may be a useful chemotherapeutic agent for the treatment of trichomoniasis.