The life-cycle of the amoeboflagellate Tetramitus rostratus includes amoeboid, cyst, and flagellate stages. The ultrastructure of these three stages is illustrated, with particular emphasis on flagellate morphology. Amoeba morphology is typical of that of limax amoebas. Cysts, forming from trophic amoebas, are enclosed by a wall made up of two layers: ectocyst (ca. 70 nm), and endocyst (200 nm). The wall apparently forms from precursor material present in vesicles in the pre-cyst stage cytoplasm. Flagellate morphology is characterized by a well-defined top-shaped profile, maintained by microtubules under the plasma membrane. The flagellar apparatus or mastigont consists of four flagella, their basal bodies, sheaves of microtubules associated with two of the basal bodies, and several rhizoplasts (periodicity 20 nm). A deep, microtubule-supported, ventral invagination appears to function as a gullet. A small number of mitotic stages observed in amoeboid and flagellate individuals suggests similarity in the division process in both stages: intranuclear mitotic apparatus, nucleolus persisting through mitosis, no centrioles or basal bodies functioning as centrioles, difficulty in resolving chromosomes. The text compares ultrastructures of several amoeboflagellate organisms and evaluates the phylogenetic significance of those features common to different species. On the basis of this study, Tetramitus most closely resembles Naegleria spp.