Lignocellulose is the most abundant natural biopolymer on earth and a potential raw material for the production of fuels and chemicals. However, only some organisms such as bacteria and fungi produce the necessary enzymes to metabolize it. In this work we detected the presence of extracellular cellulases in the genome of five species of Scenedesmus. These microalgae grow in both, freshwater and saltwater regions as well as in soils, displaying highly flexible metabolic properties. The comparison of sequences of the different cellulases with hydrolytic enzymes from other organisms by means of multi-sequence alignments and phylogenetic trees showed that these enzymes belong to the families of glycosyl hydrolases 1, 5, 9 and 10. In addition, most of these presented a greater similarity of sequence with enzymes from invertebrates, fungi, bacteria and other microalgae than with cellulases from plants; and the 3D modeling data obtained showed that both the main structures of the modeled proteins and the main amino acid residues implicated in catalysis and substrate binding are well conserved in Scenedesmus enzymes. We propose that these cellulase-producing phototrophic microorganisms could act as catalysts for the hydrolysis of cellulosic biomass fueled by sunlight.