In this paper, we examine the Swedish pre-service preschool teachers’ knowledge about the negative numbers, which is one of the central content areas in their compulsory mathematics course. Our results show that almost a half of the pre-service preschool teachers can give a reasonable definition for the negative numbers, but only a few of them set the negative numbers in relation to the other common number sets. The participants’ performance in a set of mathematical tasks related to the negative numbers follows the qualitative variance in their definitions of the negative numbers, yet this relation is not completely direct; there were also such participants who were not able to give a definition but succeeded well in the tasks. Those who gave a vague definition performed weaker than the others. Although the participants’ knowledge about the negative numbers is, perhaps, not satisfactory with every respect, it does not significantly differ from that of a control group that consists of (non-mathematician) university teachers from their study programme.