  We construct a supersymmetric model based on the semi-simple gauge group
$SU(3)_c \times SU(3)_L \times SU(3)_R$ with the relation $tan\beta \simeq
m_t/m_b$ automatically arising from its structure. The model below a scale
$\sim 10^{16}$ GeV gives naturally rise just to the minimal supersymmetric
standard model and therefore to the presently favored values for $sin^2
\theta_w$ and $\alpha_s$ without fields in representations higher than the
fundamental.
