A comparative study of selenate and selenite assimilation by Salmonella typhimurium revealed that selenite was not transported by the sulphate permease. Selenite uptake could be detected both in wild-type cells repressed for sulphate transport and in mutants that lacked a functional sulphate permease. In contrast, selenate was assimilated by the same process as was sulphate; selenate transport was repressed under the same conditions which repressed sulphate uptake and was absent in permeaseless mutants. Selenite transport was absent if cells were glucose starved or treated with either azide or p-chloromercuribenzoate. The pH optimum was between pH 6 and pH 7; transport was most rapid at 36 degrees C. The double reciprocal plot for selenite transport at different substrate concentrations was biphasic: between 10 and 50 microM SeO32(-) the apparent Km was 37.8 microM, and at higher concentrations, 2.87 mM. The transport rate for 0.1 mM SeO32(-) was significantly stimulated by sulphite concentrations up to 5.0 mM, with a maximum at 3.0 mM SO32-. The results establish a selenite transport process, in S. typhimurium, as the initial step of an assimilatory pathway selective for selenium.