Fibrostatins, potent inhibitors of prolyl hydroxylase, were isolated as orange crystals from the culture broth of strain No. 23924, which was identified as Streptomyces catenulae subsp. griseospora. In vitro inhibitory activity (ID50 value) of fibrostatins A, B, C, D, E and F against prolyl hydroxylase of chick embryos was 23, 39, 29, 180, 10 and 14 microM, respectively.