  We report on the formation of ultracold fermionic Feshbach molecules of
$^{23}$Na$^{40}$K, the first fermionic molecule that is chemically stable in
its ground state. The lifetime of the nearly degenerate molecular gas exceeds
100 ms in the vicinity of the Feshbach resonance. The measured dependence of
the molecular binding energy on the magnetic field demonstrates the
open-channel character of the molecules over a wide field range and implies
significant singlet admixture. This will enable efficient transfer into the
singlet vibrational ground state, resulting in a stable molecular Fermi gas
with strong dipolar interactions.
