Previous phytochemical studies of the genus Prosopis (Leguminosae) have demonstrated that several species contain a number of known flavonoids (1-3) as well as alkaloids (4-10). In this paper we wish to report the isolation and structural determination of N-methyleassine (1) from the bark of Prosopis nigra (Griseb.) Hieron., P. ruscifolia Griseb. and P. tinalillo Stuck. This is the first report of the isolation of this compound from a natural source, although it has been previously synthesized from cassine by Highet (11).The crude alkaloid extracts obtained from all the species, when purified by preparative tlc, yielded N-methyleassine. The mass spectrum showed the molecular ion at m/e 311 (0.79%) (C₁₉H₃₁NO₃) and the base peak at 128 (100%) due to the loss of the aliphatic side chain. Other identified peaks indicated the elimination of the COCH₃ moiety and a number of signals, separated 14 m.u. each, corresponding to the degradation of the side chain. This pattern of fragmentation is coincident with that previously reported for this type of alkaloid (8).The nmr spectrum was almost identical with that of cassine, except for a singlet at δ 2.28 (3H) attributed to the methyl attached to the nitrogen.The identification of (1) was confirmed by comparison of the physical properties with those of the authentic compound which was prepared from cassine according to the literature (11). A portion of N-methylcassine was converted in the hydrochloric derivative by treatment with 5% methanolic hydrochloric acid solution, evaporated to dryness and recrystallized from absolute ethanol. The mp observed was 109-110° (lit. (11): 110.5-111.5°).