A search for free neutron-antineutron oscillations has been carried out at the Pavia Triga Mark II research reactor. A thin carbon target is crossed by a beam of thermal neutrons propagating in a 18.5 m long channel where the earth magnetic field is attenuated by a factor of 50. The total neutron current through the target is 3.2×1010 n/s. Possible antineutron annihilations are identified by a large track detector surrounding the target. A lower limit on the oscillation time of 4.7×105 s (90% C.L.) has been reached.