The very wide field of animal behaviour has no more fascinating application than in the bull stud of any artificial insemination centre where the male is subjected to close observation over a long period of time. After a year or two's experience one becomes aware of the qualifications expected of a bull if he is to become a successful A.I. sire. His general temperament must be good, he must be placid and easy to handle, be free from all vices, and be able to produce high quality semen as and when required, and at the same time he must possess superior genetical material so that there is hope of improved production from his progeny.