This study evaluated the growth and biochemical composition of farming Gracilaria gracilis (Stackhouse) M. Steentoft, L. M. Irvine & W. F. Farnham in the Bizerte Lagoon (BL) and Bizerte Bay (BB) in the North Coast of Tunisia, using lantern nets. Effects of site and depth on alga daily growth rate (DGR) and biochemical composition were investigated. The DGR was affected by culture site (1.42 ± 0.65% day −1 and 1.19 ± 0.34% day −1 for the BL and the BB respectively). Agar yield, was higher (p < 0.05) in the BB than the BL (23.31 ± 2.64% vs. 19.19 ± 2.32%) with a higher (p < 0.05) 3,6-anhydrogalactose (3,6-AG) contents (41.37 ± 3.68% vs 23.30 ± 5.40%) and a lower (p < 0.05) sulphate degree (6 ± 2.00% vs 8.80 ± 0.86%). The proteins contents were independent of the site and depth of culture (20.74 ± 7.22% and 22.02 ± 6.34% for the BL and the BB respectively). R-phycoerythrin (R-PE) contents were significantly higher (p < 0.05) in the BB (0.86 ± 0.31 mg g −1) than those obtained in the BL (0.33 ± 0.12 mg g −1). The salinity, transparency, nitrate and ammonium were monitored in both sites, and their influences were discussed. Our results suggest that G. gracilis cultured in Bizerte Bay can be used in a cascading biorefinery approach. Global seaweed production, largely derived from aquaculture (96.5 percent by volume of the wild-collected and cultivated aquatic plants combined), has changed considerably since 2005 1. This production increased from 13 million in 2005 to reach 30 million tons (live weight) in 2016. Moreover, 99% of the world's production comes from Asian countries, notably China (47.9%), Indonesia (38.7%), Philippines (4.7%), Republic of Korea (4.5%), Democratic People's Republic of Korea (1.6%), Japan (1.3%) and Malaysia (0.7%). Furthermore, trade in aquatic plants increased from USD 60 million in 1976 to more than USD 1 billion in 2016, with Indonesia, Chile and the Republic of Korea the major exporters, and China, Japan and the United States of America the leading importers. Accordingly, red seaweed production accounts for 53% of the world production 1. The most exploited red algae, expressed in a million tons year −1 , are Euchema seaweeds nei and Eucheuma spp. (10.5), Gracilaria spp. (4.1) and Porphyra spp. (1.3). In contrast, the red alga Gracilaria spp. was barely farmed at all in 1990. However, the increase in Gracilaria spp. production, by aquaculture, is mainly due to the growing demand for agar. Consequently, Gracilaria spp. aquaculture has been initiated by many countries, in different regions of the world, such as Thailand, Chile, Vietnam, Portugal, Australia, Brazil and India 2. The red alga Gracilaria gracilis, which grow in Asian coasts, was introduced into the Mediterranean Sea 3 , and established in the lagoons 4-7. Consequently, the alga was found all year round, but is the most component of the BL flora between April and June 4. In Tunisia, the quantity of seaweeds harvested, was restricted along the Bizerte Lagoon and the Tunis Lake, which is inadequate to supply the raw material requi...