Ultrastructural localization of cellulose I-alpha and I-beta allomorphs in one microfibril from algal sources was investigated using electron microdiffraction. Both cellulose I-alpha and I-beta were characterized as one-chain triclinic and two-chain monoclinic unit cells, respectively, in agreement with previous studies. These two structures coexisted in each microfibril, alternating either longitudinally or laterally. The transition zone between the two phases was found to be the interface between adjacent H-bonded molecular sheets (i.e., 0.39-nm lattice planes).