The removal of mercury from aqueous solution by packed-bed reactors (PBRs) containing Chlorella emersonii (CCAP 211/8A) entrapped in alginate and agarose gels was studied. Reactors were constructed from chromatography columns packed with 200 gel particles 4-6 mm in diameter. The effects of variation in cell stocking density, influent mercury concentration, and medium flow rate on mercury removal were investigated. Phosphate uptake activity was abo evaluated to give an indication of cell viability. Accumulation of mercury by gel-entrapped Chlorella in packed-bed reactors was found to be only a small percentage (typically 5-20%) of that supplied to such reactors, whereas losses due to volatilization were more extensive (typically 40-90%). Levels of volatilization could be reduced by using agarose rather than alginate as the immobilization matrix. Variation in cell stocking density and flow rate enabled optimization of mercury accumulation, but levels of accumulation were always much lower than those reported in previous batch culture studies (i.e., 40-50%). Operation of reactors in a recycle mode resulted in no increase in metal bioaccumulation by the immobilized cell particles, but did increase mercury loss through volatilization. Extensive volatilization thus remains a major hindrance to the stable removal of mercury from aqueous effluents by gel-entrapped Chlorella, and modified/alternative systems worthy of further investigation are suggested.