The lactate dehydrogenase isoenzyme patterns have been studied in the axial muscles of the sharksEtmopterusandGaleus.Samples from red, intermediate and white muscle fibres were run separately on a polyacrylamide slab‐gel. Both sharks have three isoenzymes; all three are present in the red and intermediate fibres, while the white fibres contain only the two slowest‐moving isoenzymes. The red fibres of both sharks contain most of the fastest‐moving isoenzyme.The isoenzymes have a high tolerance towards urea; the slow moving isoenzyme is inhibited at about 2murea, the next isoenzyme at 4‐6 M urea, and some activity of the fast‐moving isoenzyme is still present at 10 M urea in the incubation medium. The LDH distribution in the fibre types is studied by histochemistry on frozen