Most solids expand when they are heated, but a property known as negative thermal expansion has been observed in a number of materials, including the oxide ZrW(2) O(8) (ref. 1) and the framework material Zn(x)Cd(1-x)(CN)(2) (refs 2,3). This unusual behaviour can be understood in terms of low-energy phonons(1-6), while the colossal values of both positive and negative thermal expansion recently observed in another framework material, Ag(3)[Co(CN)(6)], have been explained in terms of the geometric flexibility of its metal-cyanide-metal linkages(7). Thermal expansion can also be stopped in some magnetic transition metal alloys below their magnetic ordering temperature, a phenomenon known as the Invar effect(8,9), and the possibility of exploiting materials with tuneable positive or negative thermal expansion in industrial applications has led to intense interest in both the Invar effect and negative thermal expansion. Here we report the results of thermal expansion experiments on three magnetic nanocrystals-CuO, MnF(2) and NiO-and find evidence for negative thermal expansion in both CuO and MnF(2) below their magnetic ordering temperatures, but not in NiO. Larger particles of CuO and MnF(2) also show prominent magnetostriction ( that is, they change shape in response to an applied magnetic field), which results in significantly reduced thermal expansion below their magnetic ordering temperatures; this behaviour is not observed in NiO. We propose that the negative thermal expansion effect in CuO ( which is four times larger than that observed in ZrW(2) O(8)) and MnF(2) is a general property of nanoparticles in which there is strong coupling between magnetism and the crystal lattice.