  The interplay between magnetism and superconductivity in the newly discovered
heavy-fermion superconductor CePt$_3$Si has been investigated using the
zero-field $\mu$SR technique. The $\mu$SR data indicate that the whole muon
ensemble senses spontaneous internal fields in the magnetic phase,
demonstrating that magnetism occurs in the whole sample volume. This points to
a microscopic coexistence between magnetism and heavy-fermion
superconductivity.
