Perovskite manganites derived from LaMnO3 have recently become the subject of intensive study following the discovery of ＇colossal＇ magnetoresistance (a magnetically induced change in electrical resistance of up to several orders of magnitude) in several members of this family of compounds(1). The manganites exhibit a broad range of electronic and magnetic phases, ranging from low-resistance ferromagnetic metals to high-resistance insulators, which are extremely sensitive to variation of composition(2), temperature and pressure(3). A recent study showed that such sensitivity also extends to oxygen isotope exchange(4) : replacing O-16 with O-18 in La0.8Ca0.2MnO3 produces an unusually large change in the magnetic properties (a ZI-kelvin decrease in the Curie temperature). The magnitude of this isotope shift is evidence for the essential role played by electron-phonon coupling(5) in determining the transport properties of these materials, Here we show that this sensitivity to oxygen isotope exchange can be even more extreme. In its normal state, the compound La0.175Pr0.525Ca0.3MnO3 undergoes an insulator-to-metal transition as it is cooled below similar to 95 K, But we find that, after substituting O-18 for O-16, the compound remains an insulator down to 4.2 K, so providing a vivid demonstration of the importance of lattice vibrations in these materials.