An inward Shaker K⁺ channel identified in Zea mays (maize), ZmK2.1, displays strong regulation by external K⁺ when expressed in Xenopus laevis (African clawed frog) oocytes or COS cells. ZmK2.1 is specifically activated by K⁺ with an apparent K_m close to 15 mM independent of the membrane hyperpolarization level. In the absence of K⁺, ZmK2.1 appears to enter a nonconducting state. Thus, whatever the membrane potential, this maize channel cannot mediate K⁺ influx in the submillimolar concentration range, unlike its relatives in Arabidopsis thaliana. Its expression is restricted to the shoots, the strongest signal (RT-PCR) being associated with vascular/bundle sheath strands. Based on sequence and gene structure, the closest relatives of ZmK2.1 in Arabidopsis are KAT1 (expressed in guard cells) and KAT2 (expressed in guard cells and leaf phloem). Patch-clamp analyses of guard cell protoplasts reveal a higher functional diversity of K⁺ channels in maize than in Arabidopsis. Channels endowed with regulation by external K⁺ similar to that of ZmK2.1 (channel activity regulated by external K⁺ with a K_m close to 15 mM, regulation independent of external Ca²⁺) constitute a major component of the maize guard cell inward K⁺ channel population. The presence of such channels in maize might reflect physiological traits of C4 and/or monocotyledonous plants.