The main objective was to analyse the transition of the bronchus to the parabronchus in birds and to describe its specific structure in an integrated light microscopic, transmission electron microscopic (TEM) and scanning electron microscopic (SEM) study. Lung tissue from immature and mature quail was subjected to standard processing for paraffin light microscopy, TEM and SEM after intratracheal inflation with fixative. In transverse paraffin and Durcupan semithin sections, the partition incompletely closing the broncho-parabronchial transition has the appearance of a crest-like fold delineating the entrance to the underlying parabronchial vestibulum. The core of the entrance fold is composed of loose connective tissue with free cells, and has a well-developed blood supply and innervation. Voluminous groups of smooth muscle cells are interconnected with those of neighbouring entrance folds and the interatrial septa. On the parabronchial surface and partly on the bronchial surface the entrance fold is invested with simple cuboid epithelium consisting exclusively of granular cells with lamellar inclusions. On the bronchial surface, they pass into ciliated columnar pseudostratified epithelium. At the root of the parabronchially orientated surface, they continue into the mixed population of granular and squamous atrial cells of the parabronchus. Among the granular cells of the entrance fold, scattered epithelial neuroendocrine cells are consistently present. The three-dimensional visualization demonstrated the oval form of the entrance window with a circular field of non-ciliated cells delineating the entrance to the parabronchial labyrinthine system. The general structural pattern of the entrance fold, together with the complex system of interatrial trabecles of the parabronchi underline the multifactorial function of a complex system submitted to the skeletal, regulatory and host defense of the gas exchange tissue.