Cowpea [Vigna unguiculata (L.) Walp.] and soybean [Glycine max (L.) Merr.] plants were grown in pots and either inoculated with the vesicular-arbuscular (VA) mycorrhizal fungi, Glomus intraradices Schenck and Smith (cowpea) and G. mosseae (Nicol & Gerd.) Gerd. and Trappe (soybean), or provided with regular P fertilization (non-VA mycorrhizal plants). When plants were six to ten weeks old, roots were exposed to osmotic stress and stomatal behaviour monitored for several hours. Leaves of VA mycorrhizal cowpea had higher stomatal conductance (gS) than those of non-mycorrhizal cowpea before and after lowering soil water potential (Ψ) to −0.7 MPa with either sorbitol or macronutrient solutions. Pre-stress gs and the initial decline in gs after exposure to − 0.5 MPa sorbitol were similar in mycorrhizal and non-mycorrhizal soybean leaves. Stomatal conductance was higher in the latter after 2 h but higher in the former after 21 h. CO2 exchange rates and leaf water relations were similar in VA mycorrhizal and non-mycorrhizal soybean before and after soil Ψ was lowered. Higher gs at equal soil Ψ suggests that mycorrhizal root systems either scavenged water of low activity more effectively or influenced nonhydraulic root-to-shoot communication differently from that in non-infected root systems.