The brown shrimp Farfantepenaeus californiensis and the seaweed Ulva clathrata, both native to north-west Mexico, were co-cultured in lined ponds during 18 weeks. The aim of this study was to evaluate different stocking densities (10, 20, 30, 40 and 50 per m2) in terms of shrimp yield to see if the co-culture method results in shrimp yields suitable for commercial production. The presence of Ulva results in good water quality and allows culture of brown shrimp with low water exchange (10% weekly) and with low nitrogen and phosphorus content in discharged water. The final weight and specific growth rate (SGR) in shrimp between 10 and 30 per m2 were significantly higher (12.5–12.0 g and 4.56–4.53% day−1 respectively) than 40 and 50 per m2 (9.1 and 8.6 g, and 4.31% and 4.26% day−1 respectively). Total shrimp biomass generated in 30 or more shrimp per m2 was significantly higher (2.7–3.1 t ha−1) compared with 10 and 20 shrimp per m2 (1.0 and 2.0 t ha−1 respectively). The lowest feed conversion ratio (FCR) (0.97) was shown in the 10 shrimp per m2 case, and the highest FCR was seen with 50 shrimp per m2 (1.37). Shrimp survival ranged between 71% and 81%, where the highest mortality was shown in 50 shrimp per m2. The results show that the co-culture method can result in commercially interesting yields, suggesting that 30 shrimp per m2 is the best stocking density for co-culturing F. californiensis with U. clathrata, based on the shrimp performance.