Cadmium removal from water in the presence of Ca2+ or Mg2+ was investigated using the filamentous cyanobacterium Tolypothrix tenuis. Untreated cells of T. tenuis exhibited a low degree of selectivity with respect to the removal of Cd in the presence of Ca2+ or Mg2+. Pretreatment of cells with 0.1 M NaOH, however, greatly increased the level of their Cd adsorption. Langmuir parameters were calculated and the amounts of Cd adsorbed were compared for different concentrations of Ca2+. The results showed that NaOH pretreatment of cells increased their Cd adsorbing ability in the presence of Ca2+. Pretreatment with NaOH was also shown to be applicable to other cyanobacterial strains. It is thought that the biomass of filamentous cyanobacteria treated by NaOH could be utilized as a novel biosorbent for Cd removal, even from hard water containing a high concentration of Ca2+ and/or Mg2+.