Shoot and root growth of green foxtail [Setaria viridis (L.) Beauv. # SETVI] and yellow foxtail [Setaria lutescens (L.) Beauv. # SETLU] were compared under three moisture regimes under simulated field conditions in 1982 and 1983. An increase in water supply from 0.3 to 2.5 cm/week resulted in proportionately greater increases in the number of tillers and leaf area of green foxtail compared to yellow foxtail. In 1982, green foxtail produced approximately three times as many seed as yellow foxtail under all three water regimes, whereas under warmer conditions in 1983, green foxtail produced nearly six times as many seed as yellow foxtail under the highest water regime and more than 11 times under the lowest regime. By using a specially constructed periscope to observe root growth through clear plastic tubes implanted in the soil it was determined that the amount of available moisture had much less effect on root growth than on shoot growth of the two species. In 1982, green foxtail tended to produce more roots than yellow foxtail early in the season but as the plants matured no significant differences occurred between species. In 1983, no significant differences in total root length occurred between species at any growth stage. The roots of both species penetrated to a depth of nearly 60 cm, with the highest concentration of roots occurring at depths of 20 to 30 cm.