Equilibria for adsorption of L-glutamic acid on the crosslinked chitosan fiber (ChF-B) which has one kind of fixed ammonium group were investigated theoretically and experimentally. Adsorption of L-glutamic acid on ChF-B appeared technically feasible. The experimental equilibrium isotherm was independent of the initial concentration of L-glutamic acid but depended on the pH of the solution significantly. The significant effect of the pH in the experimental equilibrium isotherm disappeared in plots of the equilibrium amount of L-glutamic acid in the adsorbent phase vs the concentration of the neutral L-glutamic acid in the liquid phase. This suggested that the adsorption of L-glutamic acid was controlled by the acid/base neutralization reaction between neutral L-glutamic acid and the fixed ammonium group of ChF-B. A Langmuir-Freundlich type of equation was derived based on the mass action law. It was assumed that L-glutamic acid was adsorbed by the acid/base neutralization reaction between the carboxylic group of the neutral L-glutamic acid and the fixed ammonium group of ChF-B. The association number of L-glutamic acid appeared to be 2. The experimental equilibrium isotherms for any constant pH and the effect of the pH on the equilibrium amount of L-glutamic acid adsorbed on ChF-B were correlated well by the theoretical equations.