Many biologists are at present interested in populations of animals, which are studied experimentally and also by more abstract mathematical methods, but it is frequently difficult to obtain facts about actual populations living under natural conditions unless they are studied by sampling, the accuracy of which cannot always be precisely defined. The method described in this paper tells us of the actual numbers of headlice in crops of hair on individual people. From it we may learn the proportion of people who are infested at different times or places, and the number of lice on different people: also the proportion of males, females and young in the louse population. The results with material from Woolwich and from Lagos are set out in this paper. Further work is now in progress on material received from several parts of the world: this material will be studied and the results published at a later date.