We present a straightforward method to produce multiwalled carbon nanotubes using magnetic nanoparticles of Magnetospirillium magnetotacticum as catalysts. Carbon nanotube growth on these nanoparticles resulted in multiwalled carbon nanotubes of an average diameter of 13 nm showing a narrow distribution in diameter. The magnetic character of the iron-containing catalysts was exploited to generate biased growth orientations of the multiwalled carbon nanotubes during their synthesis. This magnetic bacteria-based synthetic approach represents a step forward towards synthesis-directed assembly of carbon nanotubes which is needed for easy integration of these materials into nanoelectronic devices.