Two submerged macrohphytes,Myriophyllum spicatum and Ceratophyllum demersum,were selected to investigate their changes in growth,biomass allocation and morphology under ammonium stress of 2 mg/L NH₄⁺-N lasting for 4 days and after removing ammonium stress in water column.Individual biomass,shoot height and biomass allocation of three organs (i.e.,root,stem and leaf) were determined.The results showed that during ammonium stress,shoot height of C.demersum increased compared to the control,while all above parameters of M.spicatum did not change much.When ammonium stress was removed,individual biomass of C.demersum increased significantly in comparison to that of the control,while that of M.spicatum kept unchanged.Both species showed significant higher shoot heights in treatment than in the control.Leaf biomass per unit plant weight of M.spicatum increased,while root biomass per unit plant weight decreased relative to the control after removing ammonium stress.This result suggested that ammonium stress in a short term (4 days) would not inhibit the growth of submerged macrophytes in later period in high irradiance (about 400 μmol/(m²·s)),and M.spicatum was more sensitive to ammonium stress than C.demersum.