We report observations of paraconductivity in a polycrystalline high-temperature superconductor, YBa 2 Cu 3 O 7−δ , which are consistent with the predictions of Aslamazov and Larkin for three dimensional fluctuations. We thus obtain an estimate of the coherence length, ξ O = 13.4±4.8 A ̊ , which is consistent with an earlier estimate, based in part on heat capacity measurements. We use the Ginzburg criterion to estimate H c2 (O)≈500kG.