Disinfecting hatching eggs by the use of formaldehyde vapour during the last three days of incubation is common practice in commercial hatcheries to minimise the presence of potential pathogenic microorganisms and so produce high hatchability and healthy chicks. This study was designed to investigate by the use of scanning electron microscopy (SEM). transmission electron microscopy (TEM) and light microscopy (LM), the effects of exposure to low levels of formaldehyde vapour (10.9 ppm) on the epithelial lining of the respiratory tract of hatching chicks in a commercial situation. As a prelude to this study, a control study on the development of the respiratory tract was carried out using similar techniques and it was established that by 19th to 20th day of incubation, the mucociliated cells of the entire respiratory tract of chicks were well developed. Formaldehyde fumigation however, caused destruction to the entire respiratory tract of the chicks, inducing pathological changes including clumping of cilia and microvilli, development of blebs or balloon-like structures on the cilial and microvillal walls, deciliation and desquamation of the epithelium. In addition, mucus production was also seen to be affected, with increased mucus production and changes in both the nature of the mucosubstances and distribution of the mucous cells and intraepithelial mucous glands. The morphological changes in the lining respiratory tract appeared to last until about the fourth week post-hatching, when regeneration of the lining epithelium appeared to be completed.