Relationships between baculum length, body weight and copulatory behaviour were examined in 66 species of carnivores and pinnipeds (Grand Order Ferae). Elongated bacula occur in most carnivore species of the families Ursidae, Canidae, Procyonidae and Mustelidae as well as in all pinnipeds studied. By contrast, members of the family Felidae have short bacula in relation to their body weights. Elongate bacula are found in carnivores and pinnipeds with a prolonged single intromission (PI) copulatory pattern. This finding agrees with results of a previous study of baculum length and PI copulatory patterns in primates. The enlarged baculum may serve to strengthen the penis and protect the urethra during prolonged intromissions. The distal pole of the baculum may also assist sperm transport since in some species it projects beyond the tip of the penis and probably contacts the female's os cervix during copulation. It is possible that stimulation of the female's genitalia by the baculum might also be important in mammals which are induced ovulators (e.g. Mustelidae). However, it is notable that elongation of the baculum has also occurred in some groups where females ovulate spontaneously (e.g. Canidae, Primates).