  We performed ultracold neutron (UCN) storage measurements to search for
additional losses due to neutron (n) to mirror-neutron (n') oscillations as a
function of an applied magnetic field B. In the presence of a mirror magnetic
field B', UCN losses would be maximal for B = B'. We did not observe any
indication for nn' oscillations and placed a lower limit on the oscillation
time of tau_{nn'} > 12.0 s at 95% C.L. for any B' between 0 and 12.5 uT.
