Muon-spin rotation (μSR) and torque magnetometry have been used to probe the anisotropy of an untwinned single crystal of YBa2Cu3O7-δ. The absence of twin planes allows the ratios of all three principal components of the superconducting effective mass tensor to be determined. The values for the in-plane anisotropy are in good agreement with values obtained by other techniques. The out-of-plane anisotropies as measured by μSR are also found to be in good agreement with other microscopic measurements, but somewhat lower than those resulting from torque measurements on this and other crystals.