Studies were made from September 1985 to August 1986 on the standing crop, algin and mannitol contents of three brown algae Colpomenia sinuosa, Hydroclathrus clathratus and Rosenvingea intricata growing at Shingle Island and Kilaklcarai near Mandapam. The vegetation of these species occurred between September and March with maximum biomass of plants from December to February. The standing crop varied from 0.814 to 4.250 kg wet/m² for C. sinuosa, 1.823 to 4.971 kg wet/m² for H. clathratus and 3.309 to 12.024 kg wet/m² for R. intricata and the algin content ranged from 4.7 to 14.1 %, 7.5 to 14.7 % and 10.4 to 20.5 % respectively with maximum values during December-February. The mannitol content varied from 0.5 to 2.2% in these seaweeds. There was no marked seasonal variation in the yield of algin and mannitol in these algae. The period from December to February is suitable for the harvest of these alginophytes for the production of sodium alginate.