Summary Broccoli plants were grown hydroponically in growth chambers with 20 μM Se supplied as selenate. The separate contributions of root and shoot to the volatilization of Se by plants supplied with six different levels of sulfate (ranging from 0 to 10 mM) in half-Hoagland's nutrient solution were determined. Most of the Se volatilized by broccoli plants was from the roots which volatilized about 26 times faster than the rate of shoots. The removal of the shoot markedly increased the amount of Se volatilized over the following 72 h, the detopped root attaining rates that were 20 to 30 times the rate of the intact root. Comparable results were also obtained for five additional species; rice, cabbage, cauliflower, chinese mustard, and wild brown mustard ( Brassica juncea). Part of the volatilization of Se by plants may involve microbes, i.e., bacteria. This is indicated by the fact that when prokaryotic antibiotics were added to the nutrient solution, the total rate of Se volatilization by root (broccoli) and nutrient solution was significantly decreased, much more than could be accounted for by the loss of microbial volatilization from the nutrient solution alone.