Blastocystis hominis, an anaerobic intestinal protozoan parasite of man, has a generation time (GT) in axenic culture of 8.5-19.4 h. depending on the strain tested. Average GT of the eight strains was 11.7 h. Zero growth time cell counts of 5.0 x 10(5)/ml to 2.0 x 10(6)/ml rose in 3-5 days to 1 x 19(7) cells/ml. The GT was determined for the 24-hr period during which the most rapid growth occurred; about 2% of the B. hominis cells were in division during this time. Division under the culture conditions provided was by binary fission, the usual mode for B. hominis in vitro as well as in vivo. Division times were determined also by direct observation of individual dividing cells in slide cultures. These were usually ca. 40-60 min but sometimes as low as 20 min.