A novel alginolytic bacterium Hydrogenophaga sp. strain UMI-18 that produces poly(3-hydroyxybutylate) (PHB) in the alginate-mineral salt (AMS) medium containing 1% (w/v) sodium alginate as a sole carbon source was isolated from a decayed brown seaweed litter. The yield of PHB produced by strain UMI-18 was 1.1 +/- 0.15 g/L of AMS and the PHB content in dried cell pellet was 58 +/- 4% (w/w). Glucose, fructose, galactose, mannose, mannitol, sucrose and lactose were also available for the production of PHB by strain UMI-18. The yield of PHB in 1% (w/v) carbohydrate media reached 2.03-2.24 g/L for glucose and fructose, 0.75-1.64 g/L for alginate, galactose, mannitol and sucrose, and similar to 0.15 g/L for lactose. The PHB produced by strain UMI-18 showed a glass-transition temperature (T-g) at 4 degrees C, a melting temperature at 175 degrees C, and an average molecular mass of 860 kDa. Draft genome analysis of the strain UMI-18 revealed that an alginate-assimilating gene cluster is located in contig 8 comprising 453,520 bp and the PHB-synthesis gene cluster is in contig 15 comprising 653,793 bp. (C) 2019, The Society for Biotechnology, Japan. All rights reserved.