Previous data in Egeria densa leaves demonstrated a strong inhibitory effect of Cs+ on passive K+ influx and on K+-induced, ATP-dependent electrogenic proton extrusion. In this paper we analyzed, using the same material, the effects of Cs+ on ammonium (NH4+) and methylammonium (CH3NH3+) transport in order to elucidate whether a common transport system for K+ and NH4+ could be demonstrated. The effects of Cs+ on NH4+- and CH3NH3+-induced titratable H+ extrusion (–ΔH+) and on transmembrane electrical potential difference (Em) in E. densa leaves were analyzed in parallel. All experiments were run either in the absence or presence of fusicoccin, corresponding to low or high H+-ATPase activity and membrane hyperpolarization and leading, in this material, to respectively active or passive transport of K+. The results suggest the presence in E. densa leaves of two distinct pathways for NH4+ uptake: one in common with NH4+ and (with lower affinity) CH3NH3+, insensitive to Cs+, and a second system, operating at higher H+-ATPase activity and Em hyperpolarization, strongly inhibited by Cs+ and impermeable to CH3NH3+. In agreement with this hypothesis, Xenopus laevis oocytes injected with the KAT1 RNA of Arabidopsis thaliana were permeable to K+ and NH4+, but not to CH3NH3+.