Stingless bees belonging to the Meliponinae subfamily, are known as meliponines. Scaptotrigona affinis postica Latreille, 1807 from northeast of Brazil is popularly known as tubi in Maranhao State. Scaptotrigona, which is widely distributed in neotropical regions, includes species that build their hives in pre-existing cavities. Flavones di-C-glycosides, and the pyrrolizidine alkaloid 7-(3-methoxy-2-methylbutyryl)-9-echimidinylretronecine were reported previously in propolis from S. postica. Fractions 40 AEP and 40 MEP from ethanolic extract were analyzed by LC-MS. The chromatographic profile of fractions 40 AEP and 40 MEP revealed the presence of many pyrrolizidine alkaloids, among them, lithosenine (14), lithosenine arabinoside (19), 7-angeloyl-9-(2,3-dihydroxybutyryl) retronecine (1), 7-(2-methylbutyryl) retronecine (3), 9-sarracinoylretronecine (13) and viridinatine (8), besides the flavonoids schaftoside (15), aromadendrin-7-O-methyl ether (12), 7-methoxy-5,6,3,4,5,6-hexahydroxy-flavone-3-O-glucuronide (11), mangiferin (10) and 3-O-methyl mangiferin (17). Fractions 40 AEP and 40 MEP showed antimicrobial activity against Gram positive bacteria, including Escherichia coli D31- streptomycin resistant. Cell viability was expressed in terms of the relative absorbance of treated and untreated cells (control). There was no statistical difference between treated and untreated cells.