In the lumbar region the longissimus thoracis and iliocostalis lumborum are separated by the erector spinae aponeurosis and its ventral reflection--the lumbar intermuscular aponeurosis. Lumbar fibres of the longissimus arise from the ilium and the lumbar intermuscular aponeurosis and insert into the accessory processes and proximal ends of the transverse processes of the lumbar vertebrae. Lumbar fibres of iliocostalis insert into the costal elements of the first four lumbar vertebrae. The lumbar insertions of these muscles are homologous to their thoracic insertions. The lumbar intermuscular aponeurosis is homologous to the lumbar intermuscular septum in the dog, cat and monkey. The details of attachment of the lumbar fibres of the erector spinae and of the lumbar intermuscular aponeurosis should be taken into account in biomechanical analyses of the lumbar vertebral column.