  The temperature dependence of the London penetration depth,lambda(T), was
measured in both single crystal and polycrystalline MgB_2 samples by a
high-resolution, radio frequency technique. A clear exponential temperature
dependence of lambda(T) was observed at low temperature, indicating s-wave
pairing. A BCS fit to the lowest temperature data gives an in-plane energy gap
Delta of 2.6\pm0.2 meV (2\Delta/T_c=1.5\pm0.1), which is significantly smaller
than the standard BCS weak coupling value of 3.5. We find that the data are
best described by a two-gap model.
