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The physico-chemical characteristics of the Lake Baikal ecosystem classify it as an oxidative environment. Hydrogen peroxide (H2O2) is often used for the simulation of the responses of hydrobionts to oxidative stress conditions. Here we studied the survival rate and behavioural reactions of several species of amphipods from Lake Baikal and, for reference, Holarctic Cammarus lacustris in the presence of H2O2. Preference and avoidance reactions to H2O2 were estimated in special flow installations with pairwise choices. The amphipods were ranked by their degree of preference to H2O2 (in descending order) as: Eulimnogammarus vittatus = E. verrucosus > Pallasea cancellus > Gmelinoides fasciatus ⩾ G. lacustris. The deep-dwelling Ommdtogammarus flavus showed neither preference, nor avoidance reactions to the introduction of H2O2. A direct correlation was revealed between die survival rate of amphipods in H2O2 solutions and their body size. The smaller their body size, the less resistance was demonstrated by the species: E. verrucosus > E. vittatus ⩾ G. lacustris > E. cyaneus > G. fasciatus. The results of both series of experiments gave evidence that under conditions in which an active choice was possible, G. lacustris demonstrated a higher sensitivity to H2O2, than the Baikalian species we studied. Possible relations between die reactions discovered for the Baikalian amphipods, their living environment, and their possible expansion beyond Lake Baikal are discussed [ABSTRACT FROM AUTHOR]