Abstract Ulva lactuca, a green seaweed, accumulates on beaches and shallow estuaries subject to eutrophication. As a residue, and a macro-algae, it is a source of sustainable third generation biofuel. Production of biomethane from mono-digestion of U. lactuca, however is problematic due to high levels of sulphur and low ratios of carbon to nitrogen. Fresh and dried U. lactuca were continuously co-digested with dairy slurry at ratios of 25%, 50% and 75% (by volatile solid content) in 6 number 5 L reactors for 9 months. The reactors digesting a mix with 75% U. lactuca struggled to reach stable conditions. Volatile fatty acid levels of 14,000 mg l-1 were experienced. The levels of ammonia increased with percentage U. lactuca in the mix. Optimum conditions were observed with a mix of 25% fresh U. lactuca and 75% slurry. A yield of 170 L CH4 kg-1 VS was achieved at an organic loading rate of 2.5 kg VS m-3 d-1.