SUMMARY
Coastal areas of Biak Island consist of three major ecosystems: the mangrove, seagrass beds, and coral reefs where a variety of molluscs live. Mollusc diversity in South Biak waters was investigated in September 2011. The observation was conducted at 4 locations, i.e. Paray, Ambroben, Yenures, and Sorido with 2 stations at each location. The study aimed to obtain information on the diversity and community structure of the benthic molluscs (gastropods and bivalves) on the coastal areas of South Biak waters. Sampling method applied was quadrate-transect line in intertidal areas from inshore to offshore. The molluscs (epifauna and infauna) found inside the quadrate-transect were counted and identified. The result showed high benthic mollusc diversity with a total of 94 species, consisting of 75 species of gastropods and 19 species of bivalves. The most widespread gastropod found in all stations was Nassarius sp., while for bivalves was Tellina sp. The highest value of diversity index (H) was 2.96 found in Paray 1 and the lowest was 0.58 in Yenures 1. The Evenness index (E) and Simpson index (D) ranged from 0.27 to 0.96 and from 0.06 to 0.72 respectively. Species richness (d) ranged from 2.89 to 6.84 and similarity index from 3.90 to 42.40.