  We give a superfield formulation of the path integral on an arbitrary curved
phase space, with or without first class constraints. Canonical tranformations
and BRST transformations enter in a unified manner. The superpartners of the
original phase space variables precisely conspire to produce the correct path
integral measure, as Pfaffian ghosts. When extended to the case of second-class
constraints, the correct path integral measure is again reproduced after
integrating over the superpartners. These results suggest that the superfield
formulation is of first-principle nature.
