Rational numbers and particularly fractions are difficult for students. It is often claimed that the "natural number bias" underlies erroneous reasoning about rational numbers. This cross-sectional study investigated the natural number bias in first and fifth year secondary school students. Relying on dual process theory assumptions that differentiate between intuitive and analytic processes, we measured accuracies and reaction times on fraction comparison tasks. Half of the items were congruent (i.e., natural number knowledge leads to correct answers), the other half were incongruent (i.e., natural number knowledge leads to incorrect answers). Against expectations, students hardly made errors on incongruent items. Longer reaction times on correctly solved incongruent than on correctly solved congruent items indicated that students were indeed hampered by their prior knowledge about natural numbers, but could suppress their intuitive answers.