  Within the framework of an isospin-dependent quantum molecular dynamics
model, the rotational flow in reactions of $^{58}$Fe + $^{58}$Fe and $^{58}$%
Ni + $^{58}$Ni at 40 MeV/nucleon for different impact parameters is
investigated by analyzing the mid-rapidity azimuthal distribution. The
rotational observables are also calculated semiquantitatively. For the first
time, it is found that the more neutron-rich system ($^{58}$Fe + $^{58}$Fe)
exhibits stronger rotational collective flow. This isospin dependence of
rotational collective flow is more appreciable in semi-peripheral collisions
and it is shown to mainly result from the isospin dependence of nucleon-nucleon
cross section rather than the symmetry energy. Meanwhile, it is indicated that
the rotational flow depends strongly on the impact parameter.
