We study the nematic/isotropic (N/I) phase diagram and orientational ordering in heteropolymers consisting of stiff and flexible segments by field theory. It is shown that a finite and small variance in sequence disorder is sufficient to destroy orientational ordering; in the lyotropic case, the presence of sequence disorder triples the (N/I) coexistence density difference compared with that of stiff homopolymers at similar conditions. The typical N/I domain scale predicted using nucleation theory and field theory is similar to mu m in agreement with recent optical microscopy experimental results.