The Costa Verde and Costa do Sol Regions are coastal regions of the State of Rio de Janeiro and are located in the south and north of the state, respectively. In the Costa do Sol region there is evidence of the role of the Barreiras formation as a source of arsenic input to coastal sediments and, consequently, to fish in this region when compared to other regions. The hypothesis is the existence of higher concentrations of arsenic in fish from this region when compared to fish from Costa Verde region. The limit concentration for As in fish, according to the Agência Nacional de Vigilância Sanitária (ANVISA), is 1 mg kg(-1), but it is known that organic As species such as arsenobetaine make up approximately 70 to 95 percent of fish and are non-toxic. Therefore, for an adequate interpretation of the meaning of the presence of arsenic in fish, the differentiation between inorganic, toxic, toxic and nontoxic organic species is necessary. The samples were collected by the Fundação Instituto de Pesca do Estado do Rio de Janeiro (FIPERJ) and were sent to the Fundação Oswaldo Cruz (Fiocruz), where the part referring to the fish beef was removed, and then sent to Pontifícia Universidade Católica (PUCRIO) for determination and speciation of As. The technique used was an atomic fluorescence spectrometer coupled to a hydride generator, an ultraviolet photoreactor and a high-performance liquid chromatograph (HPLC-UV-GFAFS) to determine the amount of each species of arsenic and atomic absorption spectrometry coupled to a graphite furnace (GFAAS) to determine total arsenic in the samples. Arsenic extraction tests were carried out using certified reference samples, aiming at its speciation in fish samples, and the extraction method chosen was 2 percent nitric acid (v/v) for determination of total arsenic by GFAAS, with recovery rate of up to 100 percent reported in the literature. After carrying out the analytical curves on the equipment, determinations of total arsenic and arsenic speciation were carried out. The results obtainedshowed that few samples presented arsenic concentration above the limit estimated by ANVISA of 1 mg kg(-1). Considering the samples with arsenic concentration above 0,8 mg kg(-1), it was found that 70 percent to 95 percent is in the form of arsenobetaine, emphasizing the importance of speciation in the evaluation of arsenic concentration in fish samples.