Cells of Plectonema boryanum and Anabaena flos-aquae obtained from large scale (to 15L) cultures were used to examine the cation exchange properties of killed algal cells. Since Microcoleus lyngbyaceus, one of the most abundant species in our collections from marshes, could not be obtained in large-scale liquid cultures, it was collected in algal mats from tide-marsh pannes and separated from other algae, etc. by a combination of screening, air elutriation and density-gradient centrifugation. Characterization of the exchange properties of such preparations was pursued using 45Ca for exchange measurements and Cd-EDTA or Cd-EGTA as indicators with the Cd-specific ion electrode for electrometric titration for total Ca++ and Mg++, or Ca++, respectively. Algal cell preparations exhibited a capacity to exchange Ca++ readily with ambient aqueous solutions. K+ and Na+ could displace a substantial proportion of the Ca++ at concentrations about 100-fold greater than that of the ambient Ca++. Mg++ was about 10 times as effective as the monovalent cations in displacing Ca++. Cell walls isolated from rhizomes of Spartina alterniflora exhibited only about 1/10 the affinity for Ca++ as that found for the blue-green algal preparations. The intrinsic association constant and the total capacity for cation exchange indicated that in solutions of 2×10−4M about 50% of the exchange sites are occupied. The ligand for Ca++ could be extracted in part by dilute acid, but not by Triton X-100 or by EDTA, even in the presence of Triton X-100. The extracted material appeared to be a polysaccharide. Both the sheaths of M. lyngbyaceus by themselves and cells from which they were separated exhibited Ca-exchange properties.