The materials for this paper were obtained from three sources: (a) the examination, on successive occasions during the period December 1939–April 1940, of volunteer subjects, mainly from common lodging houses in east London, prior to their treatment with pediculicides, (b) the examination, during April 1940, of large numbers of men from common lodging houses, Salvation Army hostels, night refuges and other sources, in a search for further suitable subjects for control experiments, and (c) the study of the process of infestation of shirts issued to selected volunteers and worn continuously over a period of 10 days. The material from the first source is largely selective, since infested volunteers were specifically invited; that from the second source is more representative of the population of cheap hostels, since no emphasis was laid on the presence of lice, the men being induced to submit to examination by the offer of payment. It is, however, obviously not a random sample since it represents volunteers, and neither it nor the source (a) material should be used as a measure of the frequency or degree of infestation normal to the corresponding group of the city population.