During the production of algal biodiesel, an algal cake remains and can form a feedstock for further product formation. Potential commodity by-products include an algal biomass suitable for animal feed, fermentation of the carbohydrate portion of the algal biomass to ethanol (Sandler and Murthy, 2010), or anaerobic digestion of the algal cake to biogas. Here we consider the co-production of algal biogas to demonstrate the multi-product approach (Campbell et al., 2010; Stephenson et al., 2010; Richardson et al., 2012). Stephenson et al. (2010) investigated two modes of cell disruption and assumed that only disrupted cells were digested. While the anaero­bic digestion of Spirulina platensis was similar in the presence and absence of cell disruption, disruption was essential for Scenedesmus, which also exhibited signifi­cant resistance to disruption. Collet et al. (2010) demonstrated anaerobic digestion of the algae to biogas without prior recovery of the lipid fraction for biodiesel. This approach allowed avoidance of the concentration and cell disruption steps, thereby reducing the energy and economic costs. The digestate following methane produc­tion has potential to provide a source of N and P for further algal growth. Partial recycle has been demonstrated in the Spirulina system.