In the vascular flora of the main rock-soil system known as serpentine south of Mt. Stuart of the Wenatchee Mountains of the Cascade Range (Washington, USA), leguminous species, such as members of the genus Lupinus, are rare. Populations of Lupinus laxiflorus, L. lepidus var. lobbii. and L. polyphyllus were investigated. These species occur also on non-serpentine soils. The serpentine and non-serpentine populations therefore may be regarded as edaphic ecotypes. One of the main questions is whether the effect of serpentine soil is reflected in the physiology of N-fixing Rhizobium lupini. The root systems of the three species are nodulated. There are no indications that serpentine populations have fewer root nodules than non-serpentine populations. Bacteroids of R. lupini are present in all the nodules. The strains of R. lupini all fix atmospheric nitrogen, but as compared to commercially available strains, their effectiveness is variable and in general low. Serpentine and non-serpentine strains do not differ materially. The available evidence indicates that strains of R. lupini differ physiologically, presumably in consonance with their edaphically differentiated host species, as far as nickel and magnesium are concerned. It is therefore unlikely that the rarity of Lupinus-species i on serpentine soil types is determined by the growth peculiarities of R. lupini under the prevailing habitat conditions.