The aim of this study was the validation of crab Ucides cordatus as indicator of oil pollution in mangrove areas from Guanabara Bay. For such purpose it was performed: (1) chemical analyses of PAHs in hepatopancreas and sediment samples by gas chromatography with mass spectrometer detector (GC/MS); (2) bioassays with pyrene exposure experiments and evaluation of biological responses through pyrene metabolites measurements in urine samples; (3) micronucleus assay in crab hemocytes to verify genotoxic consequences. Sediment and crab samples were collected from September 2003 to October 2004 in four mangrove areas of Guanabara Bay: Piedade, Nova Orleans, Suruí, Canal do Peteca. The mangrove of Barra de Guaratiba was chosen as reference site. It was developed a semiquantitative and nondestructive method for measurements of pyrene metabolites in crab urine using HPLC/Fluorescence. This technique presented several advantages as sensitivity (DL= 0.01(mi) g.mL- 1) and low cost. Crabs urine was sampled straight from the field in all studied sites. Beside this, it was conducted several bioassays in laboratory with crabs exposed to pyrene. During the first stages of experiments, pyrene-1-sulphate was the main metabolite produced, followed by pyrene-1-glucoside. In crabs urine from mangrove areas, the pyrene-1-glucoside was the dominant metabolite. It was found significant differences on total PAH levels of crab samples among studied sites, which ranged from 256 in Barra de Guaratiba to 67000 (mi)g kg-1 dry weight, in Suruí. Suruí and Canal do Peteca presented the highest PAHs concentrations in crab hepatopancreas and sediment samples. Besides, three and four ring compounds were more abundant than five and six ring PAHs. In sediment samples PAHs concentrations decreased with depth for all studied sites: Barra de Guaratiba ((somatório)PAH 35-200 (mi)g kg-1d.w.); Piedade ((somatório)PAH 311-615 (mi)g kg- 1d.w.); Nova Orleans ((somatório)PAH 270-1134 (mi)g kg-1d.w.); Suruí (somatório) PAH 1280-31500 (mi)g kg- 1d.w. and Peteca ((somatório)PAH 336-6000 (mi)g kg-1d.w.). Hepatopancreas samples of crabs Ucides cordatus had higher PAHs concentrations than sediment samples with bioaccumulation factors (BAF) of 0,70 to 35,00 and 0,02 to 3,00 for the bioaccumulation factors biota-sediment (BSAF). Also, it was found high correlations between pyrene metabolites in urine and PAHs levels in crab hepatopancreas and sediments. Pyrene metabolites of crab urine samples presented strong correlation (r2= 0,77) with hepatopancreas and sediment samples from each studied site. In laboratory, micronucleus assay was performed in crabs exposed to pyrene. It was found an increase of micronucleus number in haemocytes, indicating responses of genotoxicity and mutagenicity. However, the micronucleus assay was not sensitive enough for showing contamination gradient among mangrove sites, nor it was significantly correlated with PAHs levels of crabs and sediment samples. Ucides cordatus revealed an excellent bioindicator and suitable for monitoring the environmental quality in mangrove areas.