Cobalt-doped magnetite (CoxFe32xO4) nanoparticles have been produced through the microbial reduction of cobalt-iron oxyhydroxide by the bacterium Geobacter sulfurreducens. Thematerials produced, asmeasured by superconducting quantum interference device magnetometry, X-ray magnetic circular dichroism, Mo?ssbauer spectroscopy, etc., show dramatic increases in coercivity with increasing cobalt content without a major decrease in overall saturation magnetization. Structural andmagnetization analyses reveal a reduction in particle size to less than 4 nm at the highest Co content, combined with an increase in the effective anisotropy of the magnetic nanoparticles. The potential use of these biogenic nanoparticles in aqueous suspensions formagnetic hyperthermia applications is demonstrated. Further analysis of the distribution of cations within the ferrite spinel indicates that the cobalt is predominantly incorporated in octahedral coordination, achieved by the substitution of Fe2 site with Co2, with up to 17 per cent Co substituted into tetrahedral sites © 2013 The Author(s) Published by the Royal Society. All rights reserved.