We have used neutron diffraction to observe the vortex lattice of UPt 3 . This is the first such measurement in a heavy-fermion system, a superconductor below 1 K, or in a system with such a long magnetic penetration depth (6000±15 A). It also provides the first value for the pair coherence length, 111±2 A, to be obtained directly within the pairing state of UPt 3 . The lattice is oblique hexagonal with conventional quantization. Its anisotropy can be explained by considering a combination of gap and Fermi-surface anisotropies