(1) This study of the nuclear cytology of ticks was undertaken in the hope that it might assist in problems of identification. Papers published hitherto on the nuclear cytology of ticks and mites are reviewed. (2) The material studied consists of laboratory bred specimens of ten species representing five Ixodid genera. The source and identifications as well as the rearing of Ixodid ticks are described in detail. (3) The preparation of organs and eggs, and the equipment used in this study are described. (4) An exhaustive account is given of (a) male and female mitosis of all ten species, (b) male meiosis of nine species and female meiosis of one species. (5) General aspects of the nuclear eytology of ticks, such as endomitosis, spermatogenesis and karyomerokinesis, are discussed in conjunction with the material described. (6) Incidental observations on the anatomy of Ixodes testes and their accessory glands are compared with Douglas' description of D. andersoni. (7) The discussion summarises the eytologioal observations and their bearing on taxonomy and phylogeny. It is suggested that Ixodes should be given the status of a family.