The effect of different concentrations of abscisic acid on the growth of Spirodela oligorrhiza has been studied. Abscisic acid effectively permanently arrests growth at concentrations down to 10(-1) mg/l (one part per 10 million). Normal growth tends to be resumed at concentrations of 10(-2) and 10(-3) mg/l between 9 and 12 days after treatment. A concentration of 10(-8) mg/l, however, results in a significant increase in dry weight at both 9 and 12 days after introduction to the culture medium. It is suggested that the resumption of growth 12 days after treatment at those concentrations which inhibit growth up to 9 days, is due to a possible progressive inactivation of abscisic acid resulting in a lowering of its concentration to a level that is promotive. Sterile plants must be used since abscisic acid has shown no effect whatsoever in controlling growth of Saprolegnia, a water-mould of the Phycomycetes to which Spirodela apparently is the host.