Seasonal biomass variation and agar yield of G. salicornia (C. Ag.) Dawson from Oyster Bay, Dar es Salaam, and Chwaka Bay, Zanzibar, were determined, and some properties of the agar examined. Mean biomass and canopy cover values ranged from 21–60 g/m2 and 7– 20 %, respectively. The highest mean biomass and cover values were obtained during the SE monsoon. Agar yield varied from 13.7 to 30.2 % (dry weight) and was highest during the dry NE monsoon period. Alkali treatment generally reduced agar yield by 25–56 %. Gel strength of the agar gels ranged between 118 and 251 g/cm2 and was significantly higher during the NE monsoon period (t = 2.2; P = 0.04 and t = 8.9; P << 0.05) for samples collected from Oyster Bay and Chwaka Bay respectively). Mean gel strengths of native agar samples (205 ± 45 g/cm2) was about 42 % of that of the standard agar used. The 3,6-anhydrogalactose content was highest during the rainy season (37.4–44.3 %) and tended to coincide with low gel strengths. Sulphate content varied between 0.5 and 2.8 % in both populations. It was concluded that the best period for harvesting G. salicornia for agar production in Tanzania is the dry (NE monsoon) period.