  We succeeded in growing high quality single crystals of the ferromagnetic
superconductor UCoGe and measured the magnetoresistance at fields up to 34T.
The Shubnikov-de Haas signal was observed for the first time in a U-111 system
(UTGe, UTSi, T: transition metal). A small pocket Fermi surface (F~1kT) with
large cyclotron effective mass 25m0 was detected at high fields above 22T,
implying that UCoGe is a low carrier system accompanyed with heavy
quasi-particles. The observed frequency decreases with increasing fields,
indicating that the volume of detected Fermi surface changes nonlinearly with
field. The cyclotron mass also decreases, which is consistent with the decrease
of the A coefficient of resistivity.
