From the leaves of Prosopis juliflora, two new piperidine alkaloids, juliprosinene [1] and juliflorinine [2], were isolated. Their structures were established by modern spectroscopic techniques, particularly nmr. Prosopis juliflora DC. (mesquite), a member of the Mimosaceae family, is a shrub that grows abundantly in the Sind and Punjab provinces of Pakistan (1). Many plants of the genus Prosopis are known to have medicinal properties (2) and are used in folk medicine as astringents, in rheumatism, and as remedies against scorpion stings and snake bites (3-5).In view of the therapeutic importance attributed to P. juliflora, comprehensive investigations on it have been carried out by various groups of workers (6-10). From studies on fresh leaves we reported the isolation and structure elucidation of the new alkaloids juliflorine, julifloricine, and julifloridine (9,lO). In continuation of this work we report the isolation and structure determination of two new alkaloids from this plant, which have been named as juliprosinene and juliflorinine. Preliminary screening for antibacterial activity of the alkaloid juliprosinene showed activity against Escherichia coli strains 11303 HER 1024 and K- 125 HER 1037, Klebsiella pneumoniae C,HER 11 11, Pseudomonas aeruginosa PA 01 HER 1018, Staphylococcus aureus B-827, and Shigella sonnei YER HER 1048.