  Partial cubes are graphs isometrically embeddable into hypercubes. We analyze
how isometric cycles in partial cubes behave and derive that every partial cube
of girth more than 6 must have vertices of degree less than 3. As a direct
corollary we get that every regular partial cube of girth more than 6 is an
even cycle. Along the way we prove that every partial cube $G$ with girth more
than 6 is a tree-zone graph and therefore $2n(G)-m(G)-i(G)+ce(G)=2$ holds,
where $i(G)$ is the isometric dimension of $G$ and $ce(G)$ its convex excess.
