The aim of this work was to study the seasonal variations in the yield and physicochemical properties of agar of Gelidium sesquipedale. The algae w ere collected from January 2011 to December 2011, from the coast of Mostaganem, western Algeria. The overall results show that the dry biomass of Gelidium sesquipedale was constant throughout the year (22.34 ± 3.18 %), while the agar yield varied seasonally with highest values (P<0.05) in spring and summer (32.22 ± 1.7%). Regarding the composition of the agar, total sugars was 87.63% (± 6.9 %), with a maximum in winter (P<0.05), the 3,6anhydrogalactose varied between 30.65 (±2.56%) and 40.40% (±1.34%). The agar of G. sesquipedale was slightly substituted with sulphate (2.75 ± 1.37%) and pyruvate (0.32 ± 0.09%) and had a viscosity of 11.11 mPa.s (± 1.50 mPa.s.). In general, the physicochemical characteristics of the agar of G. sesquipedale are similar to those of the commercial agar, which leads to the conclusion that this alga can be a source of production of agar.