The induction of cellular stress response systems, heat shock proteinhsp70/Hsp70 and multixenobiotic transporterabcb1, by cadmium chloride (CdCl₂) was explored in amphipod species with different stress adaptation strategies from the Lake Baikal area. Based on the lethal concentrations (LC) of CdCl₂, the sensitivities of the different species to CdCl₂were ranked (24 hr LC50 in mg/L CdCl₂(mean/95% confidence interval)):Gammarus lacustris(1.7/1.3–2.4) <Eulimnogammarus cyaneus(2.9/2.1–4.0) <Eulimnogammarus verrucosus(5.7/3.8–8.7) <Eulimnogammarus vittatus(18.1/12.4–26.6). Conjugated dienes, indicating lipid peroxidation, were significantly increased after 24 hr exposures to 5 mg/L CdCl₂only in the more CdCl₂-sensitive speciesG. lacustrisandE. cyaneus. Upon treatment with 0.54 to 5.8 mg/L CdCl₂for 1, 6 and 24 hrs,hsp70transcript levels were generally more increased after the longer exposure times and in the more CdCl₂-sensitive species. Relating the CdCl₂exposure concentrations to LCx values revealed that across the species the increases ofhsp70transcript levels were comparatively low (up to 2.6-fold) at CdCl₂concentrations ≤LC50. Relativehsp70transcript levels were maximally increased inE. cyaneusby 5 mg/L CdCl₂($\hat {=}$LC70) at 24 hrs (9.1-fold increase above the respective control). WhenG. lacustriswas exposed to 5 mg/L CdCl₂($\hat {=}$LC90) for 24 hrs, the increase inhsp70was in comparison toE. cyaneusconsiderably less pronounced (3.0-fold increase inhsp70levels relative to control). Upon exposure of amphipods to 5 mg/L CdCl₂, increases in Hsp70 protein levels compared to untreated controls were highest inE. cyaneusat 1 and 6 hrs (5 mg/L CdCl₂$\hat {=}$ LC70) and inE. verrucosusat 24 hrs (5 mg/L CdCl₂$\hat {=}$ LC45). Thus, when the fold increases in Hsp70 protein levels in the different amphipod species were related to the respective species-specific LCx values a similar bell-shaped trend as forhsp70transcript levels was seen across the species. Transcript levels ofabcb1in CdCl₂exposed individuals of the different amphipod species varied up to 4.7-fold in relation to the respective controls. In contrast tohsp70/Hsp70,abcb1transcripts in CdCl₂exposed individuals of the different amphipod species did not indicate similar levels of induction ofabcb1at equal LCx levels across the species. Induction ofhsp70andabcb1genes and Hsp70 proteins by CdCl₂in the lethal concentration range shows that these cellular responses are rather insensitive to CdCl₂stress in the examined amphipod species. Furthermore, the increase of expression of these cellular defense systems at such high stress levels suggests that induction of these genes is not related to the maintenance of normal metabolism but to mitigation of the effects of severe toxic stress.