Liquid crystalline main chain polyesters containing Y-shaped mesogenes with flexible lateral substituents of different length have been investigated by X-ray diffraction and DSC experiments. The X-ray patterns of the polymers with lateral substituents longer as the methoxy group show beside the reflections at the wide angle range also small angle equatorial reflections. This is suggesting a nanofibrillar structure. The position of these reflections depends on the length of the substituent. In contrast to sanidic phases, these nematic melts are uniaxial. X-ray and DSC measurements reveal a phase transition between the "normal" nematic and the fibrillar nematic phase.