Magnetotactic bacteria are microorganisms that orient and migrate alongmagnetic field lines. The classical model of polar magnetotaxis predictsthat the field-parallel migration velocity of magnetotactic bacteriaincreases monotonically with the strength of an applied magnetic field.We here test this model experimentally on magnetotactic coccoid bacteriathat swim along helical trajectories. It turns out that the contributionof the field-parallel migration velocity decreases with increasing fieldstrength from 0.1 to 1.5 mT. This unexpected observation can beexplained and reproduced in a mathematical model under the assumptionthat the magnetosome chain is inclined with respect to the flagellarpropulsion axis. The magnetic disadvantage, however, becomes apparentonly in stronger than geomagnetic fields, which suggests thatmagnetotaxis is optimized under geomagnetic field conditions. It istherefore not beneficial for these bacteria to increase theirintracellular magnetic dipole moment beyond the value needed to overcomeBrownian motion in geomagnetic field conditions.