This paper reports part of an ongoing investigation into the understanding of irrational numbers by prospective secondary school teachers. It focuses on the representation of irrational numbers as points on a number line. In a written questionnaire, followed by a clinical interview, participants were asked to indicate the exact location of the square root of 5 on a number line. The results suggest confusion between irrational numbers and their decimal approximation and overwhelming reliance on the latter. Pedagogical suggestions are discussed. (Contains 5 figures and 1 table.)