The adsorption and deactivation of microorganisms by a new activated carbon fiber (Aqualen(R)) were investigated. Tests were conducted with Escherichia coli, Agrobacterium tumefaciens, and Saccharomyces cerevisiae, The esterase metabolic activities of the adsorbed microorganisms were measured and were found to diminish rapidly and cease completely in less than 24 hours. An adsorption column packed with Aqualen(R) was tested for bacteriostatic effects. No microbial growth was detected. An explanation of the microbial adsorption and deactivation is offered.