Models of nutrient-plankton interaction with a toxic substance that inhibits either the growth rate of phytoplankton, zooplankton or both trophic levels are proposed and studied. For simplicity, it is assumed that both nutrient and the toxin have the same constant input and washout rate as the chemostat system. The effects of the toxin upon the existence, magnitude, and stability of the steady states are examined. Numerical simulations demonstrate that the system can have multiple attractors when the phytoplankton's nutrient uptake rate is inhibited by the toxin. © 2005 Elsevier Ltd. All rights reserved.