The aim of the paper was to describe the prevalence and the role of Demodex folliculorum and Demodex brevis in the pathogenesis of chronic blepharitis.Examinations were carried out on a group of 435 people aged from 3 to 96 years. A sample of four eyelashes were taken aseptically from each individual and later studied under a light microscope. Infection of Demodex was classified on the basis of presence of mature and larval forms or after observing chitinous exuviae affixed to the studied eyelashes.It has been concluded that Demodex occurs among: 13% of the people aged from 3 to 15 years; 34% of the people aged from 19 to 25 years; 69% of the people aged from 31 to 50 years; 87% of the people aged from 51 to 70 years and 95% of the people aged from 71 to 96 years. It has been observed that 58% of the people infected with Demodex suffer from chronic blepharitis. It was also found that Demodex frequently occurs among people taking care of the elderly and among students of the Medical School.(1) Demodex folliculorum and Demodex brevis are an etiological factor in chronic blepharitis. (2) With age the prevalence of infection of Demodex rises. (3) People taking care of the elderly and students of Medical Schools are at higher risk of being infected with Demodex.