Howler monkeys (Alouatta Lacèpéde, 1799) are folivores-frugivores with flexible diets depending on conditions. Here, we compare the diets of Alouatta caraya (Humboldt, 1812) in two riparian forests (island and mainland), in Porto Rico region, Upper Paraná River, Southern Brazil. Howlers were followed from October 2004 to September 2005 in the riparian forest of a 1,050 ha island and in the continuous riparian forest on the mainland (left bank of the river). The "scan sampling" method with instantaneous samples every 15 minutes was used. Besides vines, diet breadth was similar: 17 species consumed on the island versus 16 species on the mainland. Both consumed leaves followed by fruits more than any other food type (leaves: island - 65%, mainland - 49%, fruits: island - 24%, mainland - 46%). Even though the plant Cecropia pachystachya Trécul is less abundant in the mainland it was the item most consumed in both locations all year long, which suggests its importance for the howlers. Diet also varied both seasonally and between the island and mainland, apparently following changes in local abundance of each item and due to plant phenologies.