The buildup of magnetic flocs containing plankton on a single wire by means of magnetic seeding method is studied for the axial flow configuration. The species of plankton, magnetic seeding material, and flocculant used in this experiment are Chattonella sp. , magnetite, and polyaluminum chloride (PAC), respectively. When magnetic field intensity increases, PAC concentration decreases or magnetite concentration increases, the buildup changes from one without chains similar to that of paramagnetic particles, to one with needle-like chains similar to that of ferromagnetic particles. The former is found to be adequate for the recovery of plankton.