In France, two amphibious hydrophytes of alien Ludwigia (Onagraceae) have for about the past twenty years been causing serious ecological and economic problems: L. peploides (Kunth) Raven et L. grandiflora (Michaux) Greuter & Burdet. This bacteriological and physicochemical study, focused on three different Mediterranean aquatic ecosystems, reveals, for the first time, a direct negative impact of these American invaders. During summer, while plant growth is intensive, and the appearance in the water column of anoxic conditions and production of toxic compounds may be observed, notably in L. grandiflora stands. The toxicity is linked to a proliferation of sulphate-reducing bacteria producing sulphides that are very harmful for aquatic organisms.