Isotherm measurements and structural analysis of LB films have shown that a first-order transition appears on compression of a PLA50 monolayer. Transmission electronic microscopy and atomic force microscopy have revealed the coexistence of a condensed and an expanded phase. During the growth of the condensed phase, we observed the formation and the fusion of submicrometer domains with an overheight limited to about 17 angstrom. This indicated that the expulsed lactic units associate in structured domains.