Samples of the pars costalis and pars lumbalis diaphragmae of 20 male lambs, the crossbreds of Istrian ewes and Sardinian rams, aged 2-3 months, were taken to examine the diameter of their muscle fibres, the content of connective tissue, the activity of lactic (LDH) and succinic (SDH) dehydrogenases, phosphorylase as well standard, alkaline and acid stable myosin adenosine triphosphatase (ATP-ase). In the pars lumbalis there were 30.4 % large, 46.3 % medium and 23.3 % small diameter fibres while in the pars costalis there were 12.0 % large, 55.5 % medium and 32.5 % small diameter fibres. A relatively higher amount of connective tissue was observed in the pars costalis. The activity of dehydrogenases and phosphorylase was present in all fibres and the dehydrogenase activity was intense or moderate. In the pars lumbalis there were 39.1 % strongly and 60.9 % moderately LDH active fibres and 47.3 % strongly and 52.7 % moderately SDH active fibres. As regards the ATP-ase activity the fast fibres prevail in both phrenic parts and they made 61.7 % in the pars costalis and 53.8 % in the pars lumbalis.