We investigated the introduced form of Arundo donax (from the Botanical Garden of the Academy of Sciences of the Republic of Uzbekistan), which had not been previously studied. The dynamics of alkaloid accumulation were examined across four vegetation periods (early period: plant height 15-20 cm; development period: 1-1.5 m; vigorous growth period: 4-5 m; end of vegetation: 5-6 m) and in different plant organs. The maximum alkaloid content was found in the early vegetation period. The alkaloid content in the epigeal part gradually decreased with plant growth. In the roots, a similar pattern was observed, but alkaloids re-accumulated at the end of the vegetation period (when the epigeal part withered). The total alkaloid content of the introduced form was higher than that of the wild form (from the Shaartuz region of Tadzhikistan), though the donaxine content was lower. The qualitative composition of the introduced form was more diverse and richer than the wild form. Alkaloids were mainly localized in the leaves, roots, and rhizomes, with donaxine as the main component. Thin-layer chromatography revealed approximately ten alkaloids in the plant. Five alkaloids were isolated: donaxine, the new alkaloid arundine, and three bases—base (1) (mp 109-110°C, identified as N-phenyl-β-naphthylamine, the first isolation from a Gramineae plant), base (2) (mp 178-180°C, an optically active dextrorotatory isomer of donaxaridine, the first isolation from a plant), and base (3) (mp 110-111°C). Thus, we studied the dynamics of alkaloid accumulation in the introduced A. donax and isolated five alkaloids.