Hydrocarbonoclastic bacterial communities inhabiting mangrove sediments were characterized by combining molecular and culture-dependent approaches. Surface sediments were collected at two sampling sites in Guanabara Bay (Rio de Janeiro, Brazil) and used to inoculate in vitro enrichment cultures containing crude oil to obtain hydrocarbonoclastic bacterial consortia. In parallel, in situ mesocosms (located in the Guapimirim mangrove) were contaminated with petroleum. Comparison of bacterial community structures of the different incubations by T-RFLP analyses showed lower diversity for the enrichment cultures than for mesocosms. To further characterize the bacterial communities, bacterial strains were isolated in media containing hydrocarbon compounds. Analysis of 16S rRNA encoding sequences showed that the isolates were distributed within 12 distinct genera. Some of them were related to bacterial groups already known for their capacity to degrade hydrocarbons (such as Pseudomonas, Marinobacter, Alcanivorax, Microbulbifer, Sphingomonas, Micrococcus, Cellulomonas, Dietzia, and Gordonia groups). Other strains, with high capacity for degrading hydrocarbons (aliphatic or aromatic), were related to isolates from hydrothermal vents that have not been thus far detected in hydrocarbon-contaminated sites, nor described for their ability to grow or degrade petroleum hydrocarbons. Degradation studies showed the ability of Marinobacter, Alcanivorax and Sphingomonas isolates to degrade both PAH and alkane compounds. Our results point out the rich microbial diversity of the mangroves, whose potential for hydrocarbon degradation is promising for future studies on pollutant bioremediation.