The main results of the examination of cytological preparations of the testes of the horse-louse (Haematopinus asini), of the dog-louse (Lignognathus piliferus), and of Haematopinus consobrinus may be summarized briefly. 1. In all main points the spermatogenesis of these three species of louse agrees with that described for Pediculus corporis and Pediculus capitis by the late Professor Doncaster and the present author. Miss Foot's account of the spermatogenesis of Pediculus vestimenti is criticized. 2. In the elongating spermatid of the horse-louse, the nucleolus appears for a short period as a chromatic mass adhering to the nuclear membrane, projecting partly in and partly out of the nucleus. 3. The acroblast of the horse-louse is usually a double body consisting of two ‘bun-shaped’ halves which are sometimes of equal size, and which are separated with flat sides together by a transparent achromatic layer. The ‘acroblast’ exists as such during the single spermatocyte division, and finally forms the ‘acrosome’ of the spermatid. 4. The description of the mitosome given for Pediculus corporis is substantiated by an examination of that of the dog-louse. Gatenby's description of a ‘spireme mitosome’ is criticized.