  We prove the discrete triality invariance of the N=2 NSR superstring moving
in a D=2+2 target space. We find that triality holds also in the
Siegel-Berkovits formulation of the selfdual superstring. A supersymmetric
generalization of Cayley's hyperdeterminant, based on a quartic invariant of
the SL(2|1)^3 superalgebra, is presented.
