The effect of Nigella sativa aqueous extract was evaluated against the in vitro growth of 2 different isolates of the intestinal protozoan parasite Blastocystis hominis. Different concentrations (10, 100, 500 microg/ml) of Nigella aqueous extract and metronidazole, an active standard drug for B. hominis, were incubated with B. hominis isolates in culture media at 37 degrees C. Their possible effect on B. hominnis living cell count (LCC) was assessed on Day 1, 3 & 6. The aqueous extract of N. sativa at concentrations of 100 and 500 microg/ml showed a potent lethal effect on both B. homninis isolates, but with different extent. There is no significant difference between the inhibitory effect of N. sativa and metronidazole on the LCC on the 6th day. On assessment of living cell rate (LCR) which calculate percentage rate of living cell, N. sativa at 500 microg/ml concentration has a significant inhibitory effect on both isolates. So, it is considered as the most active concentration of Nigella aqueous extract. These results prove that N. sativa aqueous extract could be useful in the treatment of B. hominis.