The synthetic DNA polymerpoly d(GC) . poly d(GC) has been studied by X-ray diffraction of orientated fibres in which it has either the well known (A or B) randed conformations have been observed with poly d(AC) . poly d(GT) and with poly d(As4T) . poly d(As4T), demonstrating that the novel structure is accessible to any DNA segment with an alternating purine-pyrimidine base sequence.