ABSTRACT. We describe the diversity, natural history and structure of snake assemblages from Marajó Island, state of Pará, Brazil, after analyzing 439 specimens deposited in herpetological collections. We tested the hypothesis that snake assem- on habit and habitat. In all, 19 species were exclusive to forest areas, 10 were exclusive to open areas and 26 species were recorded in both areas. Our results revealed greater richness for forested areas, probably due to greater habitat heterogeneity.The species composition for forested area in Marajó was similar to that found in other Amazonian assemblages, while that for open areas was more similar to the Pantanal region than other open area assemblages. The general structure of the snake assemblage of Marajó was dominated by anurophagous, terrestrial and diurnal species. Terrestrial, arboreal and semi-arboreal snakes showed a seasonal offspring production pattern, while the pattern for aquatic and semi-aquatic species was aseasonal.The structure of assemblages was influenced by both historical and ecological factors. KEY WORDS. Amazon, assemblage, functional groups, herpetofauna.GM Rodrigues et al.ZOOLOGIA 33(1): e20150020 | DOI: 10.1590/S1984-4689zool-20150020 2 / 13 produced a list of 38 snake species. Subsequently, yuki & SantOS (1996) listed 52 species of snakes for these two islands using specimens deposited in the MPEG. This set of data resulted in a list of snake species for the island but did not include details about their structure and functional groups.Given that Marajó Island includes both densely forested and open areas, we test the hypothesis that their snake assemblages are distinct with regard to structure, composition and functional groups. Additionally, we compared the snake composition of forested versus open areas of Marajó with comparable assemblages in other regions of Brazil. We provided more detailed information on natural history, including diet, reproduction and species distribution throughout the island. MATERIAL AND METHODSMarajó Island, located between the Brazilian states of Amapá and Pará (0°59'39"S, 49°44'25"W), has an area of approximately 49,000 km 2 (gOulding et al. 2003), being delimited to the south by the Pará River, to the north by the Atlantic Ocean, to the east by the Marajó Bay and to the west by the mouth of the Amazon River. Three distinct ecological regions can be found on the island: dense forests predominate in the western half; successional formations or alluvial fields cover almost its entire eastern half; and Cerrado (natural grasslands) (BritO & ilkiu-BOrgeS 2013) predominates in small areas of the central-north and south eastern regions of the island (radam 1974) (Fig. 1). Our main objective was to test if there are differences between the snakes assemblages of forested and open areas of the island. To make the comparison more robust we considered both the successional formations and the Cerrado together as comprising open habitat.There is a distinct relationship between the vegetational distribution on the island...