The phytochrome-mediated attachment of root tips of mung bean (Phaseolus aureus) and barley (Hordeum vulgare) to glass is affected by the prior exposure of hydrated seeds or seedlings to red or far red radiation. Prior irradiation of seeds or seedlings of mung bean with red light promotes attachment, while far red light promotes detachment of root tips. Similar exposure of barley seeds and seedlings to red light accentuates detachment, while far red light accentuates attachment of root tips. Red-far red light reversibility of the pretreatments indicates phytochrome control.Indoleacetic acid concentrations of 10 nM or higher appear to mimic the effects of far red light pretreatments in both mung bean and barley roots tips.