ABSTRAK Telah dilakukan penelitian dengan judul uji bioaktivitas Rf>0,5 ekstrak metanol daun mangrove Rhizophora mucronata L. (Rhizophoraceae) terhadap bakteri resisten Escherichia coli pada bulan Januari 2018. Isolasi metabolit sekunder dilakukan di Laboratorium Kimia Laut, Fakultas Kelautan dan Perikanan, serta uji bioaktivitas dilakukan di Laboratorium Mikrobiologi, Fakultas Kedokteran, Universitas Syiah Kuala. Tujuan penelitian ini ialah untuk mempelajari bioaktivitas Rf>0,5 dari ekstrak metanol daun mangrove Rhizophora mucronata sebagai antibakteri resisten Escherichia coli. Metabolit terisolasi yang aktif didasarkan pada bioaktivitas melalui kromatografi lapis tipis. Hasil uji fitokimia mengindikasikan adanya senyawa aktif alkaloid. Hasil uji bioaktivitas menunjukkan bahwa konsentrasi 20 μg/mL memiliki zona penghambat 7,50 mm dan konsentrasi 100 μg /mL memiliki zona penghambat 8,50 mm. Meskipun, diameter zona hambat 0.5 methanol extract of mangrove leaf Rhizophora mucronata L. (Rhizophoraceae) for bacteria resistent Escherichia coli was conducted in January 2018. Secondary metabolite isolation was carried out at the Marine Chemical Laboratory, Faculty of Marine and Fisheries, and bioactivity test was conducted at Microbiology Laboratory, Medical Faculty, Syiah Kuala University. The purpose of this research was to study the methanol extract bioactivity Rf >0.5 of mangrove leaf Rhizophora mucronata against Escherichia coli. Isolated metabolites active are based on bioactivity through thin layer chromatography. The phytochemical tests indicate the presence of the active compounds of alkaloids group. The bioactivity test showed at concentration of 20 μg/mL has an inhibition zone of 7.50 mm and at concentration of 100 μg/mL has an inhibition zone of 8.50 mm. Although, the diameter of inhibition zone less than 12 mm (resistant), the methanol extract of Rhizophora mucronata showed a potency to inhibit Escherichia coli growths. Keywords: Rhizophora mucronata leaf, alkaloids, bacterial resistant Escherichia coli