Phytochemical investigation of the constituents in acetone extract of the aerial parts of Mentha longifolia Linn. Hudson resulted in the isolation of five phenolic components. They were identified as rosmarinic acid [1], 4, 10-dihydroxy-3, 9-dimethoxy-pterocarpan (Melilotocarpan D) [2], Apigenin-4`-methoxy-7-O-glucoside (Tilianin) [3], quercetin-3-O-α-L-rhamnopyranosyl-(1→6)-β-D-glucopyranoside (Rutin) [4] and p-methoxy benzoic acid (p-anisic acid) [5]. Their structures have been identified by their MS and NMR spectral data and confirmed by comparison with reference values. Compounds [2-5]were isolated from the genus Mentha for the first time, and compound [1]was isolated from M. longifolia for the first time. The cytotoxic, antioxidant and antimicrobial activities of the acetone extract were evaluated.