A pilot scale fluidized bed bioreactor to control the cyanobacterium, Microcystis, was tested in an outdoor experimental pond system (28 m3) over a 57 day period. The pond system was inoculated with a wild bloom of Microcystis, and the bioreactor was preinoculated with an oligochaete, Aeolosoma hemprichi, which is known to prey on colonial Microcystis. This and other Microcystis predators such as the rotifer, Philodina erythrophthalma were observed to colonize the bioreactor during the experiment. The bioreactor performance in removing Microcystis was estimated using a mathematical model and a multiple regression analysis of the chlorophyll-a concentration, which was a satisfactory surrogate for the Microcystis cell density in the ponds. The estimated specific decrease in chlorophyll-a concentration due to bioreactor treatment was 0.04 day-1, which was equal to the net removal of 4.3 x 10(11) Microcystis cells day(-1) from the treated pond.