Fluorescent dye trans-2-[4-(dimethylamino)styryl]-3-ethyl-1,3-benzothiazolium perchlorate (DMASEBT) is a relatively recently synthesized probe for detection of amyloid fibrils accumulating in the organs and tissues of patients with a wide range of serious incurable diseases. DMASEBT was developed as an alternative of its widely used analogue thioflavin T (ThT), which is the "gold standard" for the amyloid fibrils study. Our results show the similarity of both dyes binding to amyloid fibrils and allow one to propose a mechanism of such probes interaction with some types of the fibrils. At the same time, DMASEBT has a significant advantage, namely, improved photophysical properties compared with ThT, which allows for the detection of DMASEBT-stained amyloid fibrils in the spectral region of the "transparency window of biological tissues". The ability of the dye to penetrate into the cells was shown to open the prospect of this dye's use for amyloid fibrils bioimaging and biosensing in vivo. Furthermore, it was proven that DMASEBT can be used not only as a test for amyloid fibrils formation but also for the comparative study of the fibrils structure (both their fibers and bunches), which in turn may underlie the variability of amyloidosis and affect the cytotoxicity of these protein aggregates.