The change in stomatal conductance measured soon after durum wheat (Triticum turgidum ssp. durum Desf.) was exposed to salinity was verified as an indicator of osmotic stress tolerance. It was a reliable and useful screening technique for identifying genotypic variation. The minimum NaCl treatment needed to obtain a significant stomatal response was 50 mM, but 150 mM was needed to obtain significant differences between genotypes. The response to the NaCl was osmotic rather than Na⁺-specific. Stomatal conductance responded similarly to iso-osmotic concentrations of KCl and NaCl, both in the speed and extent of closure, and in the difference between genotypes. The new reduced rate of stomatal conductance in response to addition of 50 mM NaCl or KCl occurred within 45 min, and was independent of the concentration of Na⁺ in leaves. The difference between genotypes was long-lasting, translating into differences in shoot biomass and tiller number after a month. These results indicate that the relative size of the change in stomatal conductance when the salinity is introduced could be a means of screening for osmotic stress tolerance in wheat and other cereals.