Cells of Trichomonas vaginalis (T. vag.) obtained from five patients with severe trichomoniasis were fixed for electron microscopy directly in the vaginal fluid. Samples of the same material from three of the patients was inoculated into Diamonds medium and propagated for 3–6 weeks. During the first 3–4 weeks in vitro the fine structure of all cells was changed. The shape of the nucleus, the size of the Golgi apparatus and adjacent vesicles, the fine structure of the chromatic granules, and the mastigont were all changed when compared to the organelles of the originally isolated cells. In addition the “ectoplasm” had disappeared, the glycogen granules had become sparse, and the number of free cytoplasmic ribosomes had increased. On the other hand, the large food vacuoles and the cell membranes generally were unchanged. Mycoplasms were observed in the original isolate and were also seen after more than 6 weeks of propagation, both as free organisms in the culture medium and within the large food vacuoles of “in vitro” cultured cells.