  The isomonodromic tau-function for the Hurwitz spaces of branched coverings
of genus zero and one are constructed explicitly. Such spaces may be equipped
with the structure of a Frobenius manifold and this introduces a flat
coordinate system on the manifold. The isomonodromic tau-function, and in
particular the associated $G$-function, are rewritten in these coordinates and
an interpretation in terms of the caustics (where the multiplication is not
semisimple) is given.
