  A systematic method of constructing manifestly supersymmetric
$1+1$-dimensional KP Lax hierarchies is presented. Closed expressions for the
Lax operators in terms of superfield eigenfunctions are obtained. All hierarchy
equations being eigenfunction equations are shown to be automatically invariant
under the (extended) supersymmetry. The supersymmetric Lax models existing in
the literature are found to be contained (up to a gauge equivalence) in our
formalism.
