The authors describe a rapid and sensitive magnetic relaxation switch (MRSw) aptasensor for the determination of the food-borne pathogen Pseudomonas aeruginosa. An aptamer against P. aeruginosa is covalently bound to superparamagnetic iron oxide nanoparticles. On incubation with P. aeruginosa bacteria, they will be captured by the aptamer, and this affects the formation of SPIO aggregates. The resulting strong increase in the spin-spin relaxation time (T-2) is utilized as the analytical information to quantify the bacteria. Under optimized conditions, the assay has a linear range that extends from 100 cfuai...mL(-1) to 10(6) cfuai...mL(-1), and a detection limit of 50 cfuai...mL(-1) (at an S/N ratio of 3). The method was applied to the detection of P. aeruginosa in (spiked) real food and drinking water samples and was validated by the established plate counting method. This aptasensor is considered to represent a promising platform for the determination of P. aeruginosa. Conceivably, the method may be extended to other food-borne bacteria or biomolecules for which respective aptamers are available.