New alkaloid, (E)-2-(hydroxyimino)-4-methylpentanamide (1) and a new cyclopentano[b] pyridine, 4-hydroxy-7-methyl-6,7-dihydro-5H-cyclopenta[c]pyridin-5-one (2), together with ten known compounds (3-12) were isolated from the mangrove soil-derived fungus Arthrinium sp. SCSIO 41305. Extensive spectroscopic analysis and X-Ray crystallographic analysis were used to elucidate the structure of (E)-2-(hydroxyimino)-4-methylpentanamide (1), including its absolute configuration. All the isolated compounds (1-12) were evaluated for their antimicrobial and enzyme inhibitory activities against acetylcholinesterase (AChE), neuraminidase (NAs), and phosphatidylinositol 3-kinase (PI3K). Among them, compounds 1 and 3 exhibited strong neuraminidase inhibitory activity with IC50 values of 12.04, 1.92 mu molL(-1) (IC50 20 mu molL(-1) for oseltamivir acid), while compounds 5, 6, 8, and 10 showed moderate neuraminidase inhibitory activity, and compounds 6-10 displayed weak enzyme inhibitory activities against PI3K.