Understanding the role of aspect ratio in nanomaterials for interactions with the cells is a subject of intensive research. Here, we report preparation of fluorescent polymeric nanomaterials nanorods, and study their cellular uptake for bioimaging applications. We obtained them by scission electrospun polymeric nanofibers, loaded with cationic fluorescent dyes (rhodamine or cyanines) with bulky hydrophobic counterions. The latter prevents aggregation-caused quenching dyes in the polymeric matrix and thus ensure high fluorescence brightness. The obtained nanorods of 400 nm diameter and 2-4 µm length showed good fluorescence quantum yields: 40 and 23% at 5 and 30% dye loading. Fluorescence microscopy revealed 800fold higher brightness of nanorods compared to 60-nm polymeric nanoparticles. Cellular studies highlighted dramatic differences in behavior of nanorods vs NPs. Unlike NPs, nanorods internalization depended on the cell type: while one cell line showed practically no internalization of nanorods, four others showed strong internalization. Moreover, fluorescence microscopy suggested much higher signal inside the cells for nanorods compared to NPs incubated at the same mass concentration. Quantitative optical spectroscopy of homogenized cells and flow cytometry suggested that internalization of nanorods is nearly 100-fold more efficient than that of polymeric NPs. The mass content of polymer inside the cells reached ~3 wt%, which is an exceptionally higher value. Fluorescence microscopy studies with immunostaning suggested that the internalization of nanorods takes place by endocytosis, which was confirmed by efficient inhibition of their internalization at low temperature. However, the absence of co-localization of nanorods with LysoTracker was interpreted as incapacity of the obtained hypertrophic endosomes to convert into mature lysosomes with low pH. Remarkably, nanorods were unable to enter the cells in simple buffers (like HBSS) without growth medium or after treatment by a non-ionic surfactant. These results highlight that surface properties and eventual adsorption of serum proteins could be crucial for their cell internalization. Despite the efficient internalization, nanorods showed negligible cytotoxicity according to three different types of assays: MTT, esterase activity and cell proliferation. High accumulation of ultrabright nanorods inside the cells resulted in exceptionally strong fluorescence labeling of the cells, where the signal was ~100-fold higher vs NPs. Moreover, the labelling of cells remained strong for at least 6 days. This unique property of nanorods was applied for single cell tracking in animal tissues, where strong fluorescence signal is particularly important. We showed that two cell populations labelled with nanorods operating in two near-infrared spectral regions (based on Cy5.5 and Cy7.5 dyes) could be tracked at the single cell level under the chicken skin ex vivo. Overall, we report preparation of fluorescent polymeric nanorods that exhibit exceptional capacity accumulate inside the cells. This allows highly efficient fluorescence labelling of cells that by far outperforms that of NPs, and thus opening the route for robust imaging and long-term tracking of cells in biological systems.