Students have a great deal of difficulties learning about rational number concepts, as they are confounded by misapplying reasoning about natural numbers to fractions and decimals, referred to as a natural number bias. For example, students often think that the number of digits of a decimal, or the size of the component numbers of fractions, is enough information to determine the magnitude of rational numbers. As well, students have trouble understanding that there is an infinite number of numbers between any two fractions or decimals. Using multigroup latent class analysis, the present study examines the structure of 611 Finnish and Flemish students' rational number knowledge in order to determine the similarities and differences between these two sub-samples. Results reveal that, while the Flemish students performed somewhat better, there were only relatively minor differences in the structure of the two sub-samples' rational number knowledge. In general, it appears that the natural number bias affects these Finnish and Flemish students' knowledge of the size and density of fractions and decimals in similar ways.