An extensive literature reports that Cs+, an environmental contaminant, enters plant cells through K+ transport systems. Several recently identified plant K+ transport systems are permeable to Cs+. Permeation models indicate that most Cst uptake into plant roots under typical soil ionic conditions will be mediated by voltage-insensitive cation (VIC) channels in the plasma membrane and not by the inward rectifying K+ (KIR) channels implicated in plant K nutrition. Cation fluxes through KIR channels are blocked by Cs+. This paper tests directly the hypothesis that the dominant KIR channel in plant roots (AKT1) does not contribute significantly to Cs+ uptake by comparing Csf uptake into wild-type and the akt1 knockout mutant of Arabidopsis thaliana (L.) Heynh. Wild-type and akt1 plants were grown to comparable size and Kt content on agar containing 10 mM Kt. Both Cs+ influx to roots of intact plants and Cs+ accumulation in roots and shoots were identical in wild-type and akt1 plants. These data indicate that AKT1 is unlikely to contribute significantly to Cs+ uptake by wild-type Arabidopsis from 'single-salt' solutions. The influx of Cs+ to roots of intact wild-type and akt1 plants was inhibited by 1 mM Ba2+, Ca2+ and La3+, but not by 10 muM Br-cAMP. This pharmacology resembles that of VIC channels and is consistent with the hypothesis that VIC channels mediate most Cs+ influx under 'single-salt' conditions.