High transpiration efficiency (TE) is important for water conservation in semi-arid regions. Grain sorghum (Sorghum bicolor L. Moench) is one of the most important crops grown in dry regions. However, the stomatal resistance of sorghum lines selected by breeders for their transpiration efficiency has not been measured. Do lines with high transpiration efficiency have a higher or lower stomatal resistance compared to lines with low transpiration efficiency? To answer this question, we grew in a greenhouse eight lines of sorghum (four with high TE and four with low TE) in pots with a commercial potting mix under well watered and dry conditions, three pots per treatment. Plants in dry soil were watered at planting (6 Dec. 2006) and then not watered again. Stomatal resistance measurements were taken 10 times between 3 Jan. and 22 Jan. 2007 using a new steady state diffusion porometer recently on the market (Model SC-1, Decagon Devices, Inc., Pullman, WA). Measurements were stopped when plants in the dry pots were wilting and dying. The four lines with low TE were: PI257309 (from Argentina), PI295121 (Australia), PI586381 (Cameroons), PI267532 (India). The four lines with high TE were: PI567933 (Beijing, China), PI391652 (Shaanxi, China), PI533946 (India), PI584085 (Uganda). When all dates and replications were averaged together, the average stomatal resistances (+ standard deviations) of low TE lines under well watered and dry conditions were 19.6+3.7 and 17.3+4.5 s/cm, respectively; for the high TE lines, these values were 20.7+5.4 and 25.9+4.4 s/cm, respectively. There was great variation in stomatal resistance values. But the data indicate that the low and high TE lines have similar stomatal resistances under well watered conditions and that, under drought, the low TE lines keep their stomata more open than do high TE lines.