It has recently been suggested (Diplock & Haslewood, 1964) that certain lower vertebrates such as elasmobranchs and crocodylians are lacking in ubiquinone. Also, in a study of the lesser-spotted dogfish (Scyliorhinus caniculus) (Class Elasmobranchii, Order Selachii) Diplock & Haslewood (1965) were unable to demonstrate the biosynthesisof ubiquinone. In view of the fact that in all other vertebrate species so far examined the presence of ubiquinone is well authenticated (Crane, 1965), including in such a closely related animal as the shark (Nazir & Magar, 1964), these results seemed somewhat surprising. However, in view of these findings it seemed that a study of the respiratory enzyme systems of these organisms might be worth while. As a preliminary to this work an investigation on the alleged absence of ubiquinone from elasmobranch tissues was carried out.