The application of hydrophobic magnetite seeding to concentrate hematite ultrafines (<10 µm) in slimes using a high gradient magnetic separator (HGMS) with coarse expanded metal matrices was investigated. It was shown that the selective aggregation at high shear rate of the seeded hematite particles (produced by surface treatment with sodium oleate and colloidal magnetite) could significantly enhance the HGMS capture of hematite ultrafines. The mechanism of selective aggregation was discussed in terms of the combination of hydrophobic and magnetic interactions.