  A level-one representation of the quantum affine superalgebra
$\U_q(\hat{\frak{sl}}(M+1|N+1))$ and vertex operators associated with the
fundamental representations are constructed in terms of free bosonic fields.
Character formulas of level-one irreducible highest weight modules of
$\U_q(\hat{\frak{sl}}(2|1))$ are conjectured.
