  The equilibrium phase diagram of a 1:1 symmetrical mixture composed of
oppositely charged colloids is calculated using Monte Carlo simulations. We
model the system by the DLVO effective interaction potential. The phase diagram
is similar to that of its atomic analog (the ionic fluid), where a liquid-gas
first order transition emerges in the low $T-\rho$ regions being stable with
respect to crystallization. As in the ionic fluids, we have found two different
crystals: at high $T$ the fluid crystallizes in a FCC lattice, whereas at low
$T$, the liquid coexists with a BCC crystal. The region of gas-liquid stability
is observed to be narrower as the interaction range is diminished.
