  We present preliminary results of an exploratory lattice study of the vector
form factor $f_1(q^2 = 0)$ relevant for the semileptonic hyperon decay
$\Sigma^{-} --> n l \nu$. This study is based on the same method used for the
extraction of $f^+(0)$ for the decay $K^0 --> \pi^{-} l \nu$. The main purpose
of this study is to test the method for hyperon form factors in order to
estimate the precision that can be reached and the importance of SU(3)-breaking
effects.
