We present the results of an investigation of the de Haas-van Alphen (dHvA) effect in the heavy fermion superconductor UPt3. Oscillations composed of up to 8 frequency components, corresponding to cyclotron orbits in a plane normal to the a-axis, have been detected in a high purity single crystal and a study of their amplitude as a function of temperature and magnetic field has been performed in the intervals 20-150 mK and 40-115 kG, respectively. From this study we obtain estimates of the cyclotron masses, found to range approximately from 25 to 90 times the bare electron mass, and of the mean free path, found to be in excess of 1000 Å. The relationship between these findings and the results of conventional energy band calculations is discussed. © 1987.