  Based on our previous studies of the BRST cohomology of the critical N=2
strings, we construct the loop measure and make explicit the role of the
spectral flow at arbitrary genus and Chern class, in a holomorphic field basis.
The spectral flow operator attributes to the existence of the hidden `small'
N=4 superconformal symmetry which is non-linearly realized. We also discuss the
symmetry properties of N=2 string amplitudes on locally-flat backgrounds.
