Industrialization often causes polycyclic aromatic hydrocarbon (PAH) and heavy metal contamination of soil and water. In this study, we isolated a bacterium from bottom mud water around a park of Kawasaki Port, Japan, that degrades the 5‐ring PAH dibenz[a,h]anthracene (DBA). The strain, Comamonas sp. 3ah48, degraded 29% of DBA (30 μg ml−1) in 7 days, and the degradation level increased drastically, to 59%, by the addition of glutamate to the medium. The strain also degraded 40, 14, 15 and 19% of pyrene (Pyr), benzo[b]fluoranthene (BbF), benzo[k]fluoranthene (BkF) and benzo[g,h,i]perylene (BghiP) respectively. Benzo[a]pyrene (BaP) was degraded only when glutamate was added to the medium. Strain 3ah48 retained its degradation levels in the presence of 2 mmol l−1 Co2+, Zn2+ or Cr2+, at almost the same level as that without metal, and increased the DBA degradation level to 57% in the presence of 2 mmol l−1 Cu2+, suggesting the possibility of the presence of laccase. Significance and Impact of the Study Sixteen polycyclic aromatic hydrocarbons (PAHs) are listed as priority pollutants by the United States Environmental Protection Agency (USEPA). Information about the biodegradation of one of those PAHs, dibenz[a,h]anthracene (DBA), is limited. The present study focuses on DBA degradation by Comamonas sp. 3ah48 strain isolated around Kawasaki Port, Japan. Comamonas sp. 3ah48, cultured with the addition of glutamate to the medium, was found to increase the degradation level of DBA and to degrade DBA even in the presence of high concentrations of heavy metals.