QUANTUM OSCILLATIONS IN URU2SI2

We present results of studies of the quantum oscillatory magnetization and resistivity in URu2Si2. Evidence has been found for the existence of several pockets of the Fermi surface, but the major and thermodyna mically important sheets remain unobserved. The observed quasiparticle masses are of moderate magnitude, as expected from the linear coeffic ient of the heat capacity, and roughly uniform, in contrast to the iso structural cerium analogue, CeRu2Si2, in which both light and heavy qu asiparticles coexist. For one Fermi surface we have observed oscillati ons which appear to extend below the upper critical field into the sup erconducting mixed state.