A polytype phenomenon, i.e., a polymorph of layer stacking structure, has been discovered for a side-chain-type ferroelectric liquid-crystalline polymer. The samples cooled from the isotropic state to room temperature at only slightly different rates (0.2 and 0.1 C/min, for example) showed quite different x-ray diffraction patterns at low Bragg angle, from which the repeating period of layers constructed by mesogenic side groups was evaluated as 68.4 [angstrom] for the S1 phase obtained at the cooling rate 0.2 C/min and 136.8 [angstrom] for the S2 phase at 0.1 C/min. The wide-angle x-ray diffraction and infrared/Raman spectral data were almost the same between these two samples, indicating they take essentially the same inner-layer packing structure. That is, these two samples are different only in their layer stacking structure, i.e., a phenomenon of polytype. The S1 and S2 phases are speculated to be single- and double-layered structures, respectively, where the one layer or the two layers make a repeating period along the normal to the sample plane. A possibility of ferroelectric and antiferroelectric structures has been discussed for the S1 and S2 phases, respectively. By measuring the temperature dependence of the X-ray diffraction, a complicated phase transition has been found to occur amongmore » many types of phase with various repeating periods of layers: S1(68.4 [angstrom]) at 80 C [yields] S4(62.8 [angstrom]) and/or S3(218.4 [angstrom]) at 90 C [yields] S[sub C]*(64.0 [angstrom]) at 100--140 C [yields] S[sub A](68.8 [angstrom]) at 155 C [yields] Ch at 165 C [yields] isotropic where S3 is considered to be a triple-layered phase and S[sub C]*, S[sub A], and Ch are smectic C, smectic A, and cholesteric phases, respectively.« less