  An \emph{arithmetic read-once formula} (ROF for short) is a formula (i.e. a
tree of computation) in which the operations are $\{+,\times\}$ and such that
every input variable labels at most one leaf. We give a simple characterization
of such formulae. Other than being interesting in its own right, our
characterization gives rise to a property testing algorithm for functions
computable by such formulae. To the best of our knowledge, prior to our work no
characterization and/or property testing algorithm was known for this kind of
formulae.
