  We present measurements the field and temperature dependence of the
penetration depth (lambda) in high purity, untwinned single crystals of
YBa2Cu3O6.95 in all three crystallographic directions. The temperature
dependence of lambda is linear down to low temperatures, showing that our
crystals are extremely clean. Both the magnitude and temperature dependence of
the field dependent correction to lambda however, are considerably different
from that predicted from the theory of the non-linear Meissner effect for a
d-wave superconductor (Yip-Sauls theory). Our results suggest that the
Yip-Sauls effect is either absent or is unobservably small in the Meissner
state of YBa2Cu3O6.95.
