The C=O stretching band of the alpha-crystal of PLLA has been investigated. In the a crystals splitting reveals the existence of an ordered arrangement of transition moments in the different sites along the helical chain of PLLA. In the alpha' crystals splitting is not observed denoting a lack of ordered arrangement in transition moments. In the a crystals the band at 1776 cm(-1) arises from the A mode, and because of its low intensity, its profile is barely changed by transition dipole coupling (TDC) interactions. The E-1 mode is centered at about 1759 cm(-1), and is strongly split due to TDC interactions (also referred as Davydov or exciton splitting) in the lateral direction, resulting in the broad absorption profile observed below 1776 cm(-1) in the C=O stretching region. Five different sites (characterized by the arrangement of transition moments) call be discerned along the PLLA helix in the unit cell, each of them establishing TDC interactions of different strength with the chains in the First layer. The C=O stretching band can be regarded as a continuum of adjacent contributions, from which only the overall envelope is observed. The value obtained for the transition moment suggests that excitonic interactions may occur in a collective mode involving the five distinguishable C=O groups in PLLA, explaining the large splitting observed in the C=O stretching region of crystalline polylactides. Repulsive C=O dipole-dipole interactions have also been found, related to the symmetry of the unit cell.