Employing an in vitro maintenance system, in which 8-day-old Hymenolepis diminuta survives for 24 hr (Fioravanti and MacInnis, 1976), it was found that farnesol or farnesal supplementation of the medium had no beneficial effects on maintenance and these substances induced necrosis at higher concentrations. Similar experiments utilizing Schiller's (1965) culture system demonstrated that neither farnesol, farnesal, nor farnesyl methyl ether exhibited growth promoting effects and were toxic to the worms at higher concentrations. In addition, neither the 2-cis, 6-trans nor the 2-trans, 6-trans-isomers of farnesol promoted growth in the Schiller system and at higher concentrations resulted in severe necrosis within 24 hr.