The Cs(I) biosorption characteristics of Pseudomonas alcaligenes biomass was examined as a function of initial pH, contact time and initial metal ion concentration. FTIR spectra showed that the principal functional sites taking part in the sorption process included carboxyl, hydroxyl groups and -CH3 stretching groups. The pseudo-second-order kinetic model was found to be well suited for the entire adsorption process of Cs(I) on Pseudomonas alcaligenes, which indicated the biosorption process operated through chemisorption mechanism. Adsorption equilibrium studies showed that Cs(I) adsorption data followed the Langmuir model, the maximum binding capacity of Cs(I) according to Langmuir isotherm were 81.05mg/g at pH 7.0 , shaker speed 150 rpm, at 27°C and for 60 min. The present study indicated that Pseudomonas alcaligenes biomass may be used as an inexpensive, effective and easily cultivable biosorbent for the removal of Cs(I) ions from environmental and industrial wastewater.