Structured and fully bio-based polymer assemblies based on chitosan micro- and nano-particles and poly(lactic acid) (PLA) were developed using a continuous cast-film extrusion process. The microstructure and thermal properties of the resulting biocomposite films are studied. Dispersion and size distribution of chitosan particles within the films were analyzed by optical microscopy and laser diffraction. Results show a homogeneous dispersion with no particles agglomeration, due to favorable physico-chemical interactions between chitosan particles and PLA and weak particle cohesion within the agglomerates. Differential scanning calorimetry experiments evidence a pronounced nucleating effect as well as an enhanced crystal growth rate, and a great increase in crystallinity of PLA in the presence of chitosan particles. Furthermore, in the case of chitosan nanoparticles, no reduction of PLA molecular weight occurred as revealed by gel permeation chromatography. The dispersion of nanosized chitosan particles in PLA thus appears to be an efficient way to control its crystallization behavior without degrading its molecular characteristics. POLYM. ENG. SCI., 59:E350-E360, 2019. (c) 2018 Society of Plastics Engineers