Two species of lice (Pediculus capitis and P. corporis) from various areas of Russian and adjacent republics were examined and analysed for their specific features. Each species was characterized in terms of its morphology, karyology, biology, reproductive isolation, parasitic connections, epidemiology and role as casual organisms. The two species live apart on the same host and they can live on it simultaneously. P. corporis has a higher influence on the host's metabolism than P. capitis; P. corporis seems to be a younger species and its adaptation to man has not achieved the level that is inherent in P. capitis. The head lice (P. capitis) do not affect an epidemic process in typhus and so pediculosis should be identified by the species of lice.