A wide variety of microorganisms produce lipids of which filamentous ascomycetes fungi have the advantage of directly using cellulosic by-products as primary carbon source to produce them. The capacity of Penicillium to accumulate mycelial lipids has been recognized for decades. But, their potential as lipid producer from cellulosic substrates is limited. This study deals with the lipid production from a cellulosic substrate (seaweed bagasse) by the new isolate Penicillium decumbens M11. This fungus produces 6 mg of total lipids per gram of seaweed bagasse (mg/g). When a 23 factorial design was applied to study the effects of temperature, water fraction, and C/N ratio on lipid production, the fungus increased their lipid accumulation to 97 mg/g at 28°C, a C/N ratio of 50 and a 50% water fraction. Palmitic, oleic, linoleic, and stearic acids were the most abundant fatty acids.