Biosorption of heavy metals can be an effective process for the removal and recovery of heavy metal ions from aqueous solutions. In this paper uranium biosorption by protonated, Ca-pretreated and non-pretreated Cystoseira indica algae biomass was investigated in a batch system. The results of the kinetic studies showed that the sorption of uranium on protonated and non-pretreated biomass followed pseudo-second order kinetics. The effect of pH on the equilibrium uranium sorption capacity of Cystoseira indica exhibited that highest uptake occurred at pH 4 at a solution with 350 mg/l uranium concentration. At various initial uranium concentrations from 50 to 1000 mg/l, batch sorption equilibrium at 30ºC was reached within 3 hrs and the sorption isotherms were interpreted in terms of the Langmuir and Freundlich models. Equilibrium data fitted very well to Langmuir model for all studied forms of Cystoseira indica algae. The Freundlich isotherm cannot fit as well as the Langmuir model the equilibrium data of protonated and non-pretreated Cystoseira indica algae. The maximum uranium adsorption capacity on the Ca-pretreated, protonated and non-pretreated Cystoseira indica algae predicted by Langmuir isotherm at pH 4 and 30ºC was 454.5, 322.58 and 224.67 mg/g respectively.