This research was conducted to elucidate the mechanisms in water stress tolerance of Oryza glaberrima Steud. with special reference to stomatal conductance. Leaf photosynthesis (LPS) of O. glaberrima Steud. with developing soil water deficiency, was still maintained above zero when the leaf water potential decreased to as low as -2.0 MPa, while in O. sativa L. LPS became zero at -1.6 MPa. Stomatal conductances also changed in parallel with LPS in both species, suggesting that the different response of LPS to reduced leaf water potential is due to the different stomatal response between two species. In the relationship between the stomatal conductance and abscisic acid (ABA) content in the leaves, Saka, a line of O. glaberrima Steud., showed a slower decrease of stomatal conductance with increase of ABA than Nipponbare, a cultivar of O. sativa L. This attenuated response of stomatal conductance to leaf ABA content in O. glaberrima Steud. was supposed to be the cause of maintenance of higher LPS than in O. sativa L. under low leaf water potential conditions. The Rubisco content showed no significant difference in the response to developing low leaf water potential between the two species.