MATERIALS exhibiting giant magnetoresistance (GMR) undergo a large change in electrical resistance in response to an applied magnetic field. This effect is of technological interest as it can be exploited for the sensitive detection of magnetic fields in magnetic memory devices. A range of compounds have now been found to exhibit intrinsic GMR-these are all perovskites based on manganese oxide(1-4). Here we report the observation of GMR in Tl2Mn2O7, which has a pyrochlore structure and thus differs both structurally and electronically from perovskites. At 135 K the magnetoresistance ratio (the change in resistance) reaches -86% at 7 tesla, comparable to the GMR response of perovskite materials. In contrast to the hole-doped perovskites, the charge carriers in our material are electrons, as determined from measurements of the Hall coefficient. The discovery of GMR in a second class of material expands the options for optimizing magnetoresistive properties for specific technological applications.