Polyaniline-dodecylbenzenesulfonate (PANI-DBSA) fibrils prepared directly during emulsion polymerization or by precipitation from solution have been studied by transmission electron microscopy. High-aspect-ratio fibrils are obtained with typical dimensions of order 1 mm by 1 mum. The small crystallites within the fibrils are oriented with c axis parallel to the fibril axis. The crystal structure of the PANI-DBSA crystallites has been determined as orthorhombic with two PANI chain repeat units and two DBSA molecules in the unit cell; the lattice parameters are a = 1.178 nm, b = 1.791 nm and c = 0.716 nm. Based on this structure, the calculated density is 1.12 g cm-3. The systematic absences in the PANI-DBSA diffraction data are consistent with those expected for the Pmnm space group.