  Extending the gauge-invariance principle for $\tau$ functions of the standard
bilinear formalism to the supersymmetric case, we define ${\cal N}=1$
supersymmetric Hirota bilinear operators. Using them we bilinearize
supersymmetric nonlinear evolution equations. The super-soliton solutions are
discussed. As a quite strange paradox it is shown that the Lax integrable
supersymmetric KdV of Manin-Radul-Mathieu equation does not possesses N
super-soliton solution for $N\geq 3$ for arbitrary parameters. Only for a
particular choice of them the N super-soliton solution exists.
