  Within a BRST formulation, we determine the expressions of the consistent
anomaly for superstrings with extended worldsheet supersymmetries of rank N. We
consider the O(N) superconformal algebras up to N=4, as well as the `small N=4'
superalgebra. This is done using a superfield formalism, allowing to recover
previous results that were expressed in components. Moreover, we identify the
`small N=4' algebra as the constrained `large N=4' via a self-duality like
condition in superspace.
