The flood/drain reactor consists of a bed of intermediate-sized coal (roughly 0.5 to 5 mm) periodically fluidized by pumping in a mixed bacterial culture. Metabolic products and coal fines are washed out of the bed, and large pyritic inclusions and other dense mineral matter tends to settle to the base of the bed. Draining the bed then draws in fresh air for subsequent pyrite removal by microorganisms. Experiments on a laboratory-scale bed, 10.2 cm diameter with two 12 x 30 mesh Illinois #6 coals and a 1/4 in. x 28 mesh Pittsburgh #8 coal showed that up to 80% pyrite removal could be achieved in four weeks by this combination of physical separation and microbial activity. Operational problems included removal of fines from the bed during start-up and the appearance of acidophilic protozoa grazing on the pyrite-oxidizing bacteria. Significant improvement in performance is anticipated by eliminating the lag-phase of the sulfur-oxidizing bacteria and optimizing the timing of the flood/drain cycles.