Single-crystal fibers of Bi12SiO20 and Bi12TiO20 up to 90 mm in length with diameters of 200-1200 pm have been pulled along the <001> and <011> crystallographic axis by the LHPG method. The powder X-ray diffraction and the electron-probe microanalysis have been used for characterization of the as-grown fibers. Microstructure of the sillenite fibres has been studied by the chemical etching method. The dislocations, inclusions and striations have been found to be the main imperfections in the as-grown fibers. The dislocation density has been estimated on the ground of the etching pits using optical and electronic microscopy and compared with that evaluated by the Tsivinskii's relation. The conditions of formation of microdefects are discussed in relation with the growth conditions, morphology and chemical composition of the as-grown fibers.