Type 2 and 4 transparent and opaque Neisseria gonorrhoeae demonstrated a logarithmic loss of viability with a half life of approximately 10-30 min when incubated in the presence of Trichomonas vaginalis. Although this effect was observed in the absence of serum for most types of gonococci tested, it was consistently enhanced by the addition of human serum. Only for type 4 transparent gonococci did this process show an absolute serum requirement. Cytochalasin B inhibited the loss of viability. The nonphagocytic cattle parasite Tritrichomonas foetus did not ingest or kill N. gonorrhoeae. Electron microscopy revealed phagocytic uptake and degradation of N. gonorrhoeae in T. vaginalis, indicating that the loss of viability of N. gonorrhoeae was the result of phagocytosis followed by intracellular killing of gonococci by T. vaginalis.