  We have studied the ground state phase diagram of a spin ladder with
alternating rung exchange $J^{n}_{\perp} = J_{\perp}[1 + (-1)^{n} \delta ]$ in
a magnetic filed, in the limit where the rung coupling is dominant. In this
limit the model is mapped onto an $XXZ$ Heisenberg chain in a uniform and
staggered longitudinal magnetic fields, where the amplitude of the staggered
field is $\sim \delta$. We have shown that the magnetization curve of the
system exhibits a plateau at magnetization equal to the half of the saturation
value. The width of a plateau scales as $\delta^{\nu}$, where $\nu =4/5$ in the
case of ladder with isotropic antiferromagnetic legs and $\nu =2$ in the case
of ladder with isotropic ferromagnetic legs. We have calculated four critical
fields ($H^{\pm}_{c1}$ and $H^{\pm}_{c2}$) corresponding to transitions between
different magnetic phases of the system. We have shown that these transitions
belong to the universality class of the commensurate-incommensurate transition.
