Prosopis glandulosa Torr. var. torreyana (L. Benson) M. C. Johnston root bark, collected in Texas, yielded two new alkylated piperidine alkaloids, namely 3-isocassine (1) and N-methyl-3-isocassine (2), together with the previously reported N-methylcassine (3). In addition, the alkylated piperidinyl indolizidine alkaloid juliprosopine (4) was isolated from the leaves of the same plant. The structures and stereochemistry of compounds 1-4 were determined from 1D and 2D NMR data, including COSY, HMQC, HMBC and NOESY experiments, and ESI-HRMS. Juliprosopine (4) demonstrated antimicrobial activities against a panel of microorganisms, including potent antifungal activities against Aspergillus fumigatus.