The shape of the nucleus of blood heterophils was examined in 67 reptilian species. Two distinct forms of nucleus were found, either unlobed or lobed. In all species of Chelonia, Crocodylia and Serpentes examined the heterophil nucleus was unlobed whereas, in the Lacertilia, unlobed nuclei were found in the anguinomorph families Anguidae, Helodermatidae and Varanidae but lobed nuclei occurred in the heterophils of the scincomorph families Scincidae, Cordylida, Teiidae and Lacertidae and in the more primitive lacertilian families Iguanidae, Agamidae, Gekkonidae and Chamaeleonidae. These differences have been related to possible phylogenetic, morphological and haematological factors.