Actin was located in tachyzoites of Toxoplasma gondii by indirect immunofluorescence using anti-actin antibodies. Fluorescence was seen in the region of the conoid of almost all cells. In about 75% of the cells, the whole anterior region showed fluorescence and in about 25% of the cells fluorescence was seen in the posterior region. No fluorescence was seen when permeabilized parasites were incubated in the presence of NBD-phallacidin which binds to F-actin. This observation, associated to the fact that no microfilaments were seen in transmission electron micrographs of Triton X-100 extracted, tannic acid-glutaraldehyde (TA-GA) fixed cells, indicate that in tachyzoites of T. gondii actin is predominantly in the monomeric form (G-actin). No fluorescence was observed when permeabilized parasites were incubated in the presence of antibodies specific to desmin, vimentin and keratin. Examination of Triton X-100 extracted, TA-GA fixed parasites showed that the outer membrane was partially removed while the inner membrane complex was not, but had a corrugated aspect. Connections between the sub-pellicular microtubules and the inner membrane complex and between the latter and the outer membrane were seen.