ABSTRACTIn connection with population work on lampreys at Loch Lomond, a variety of methods of marking ammocoete larvae were evaluated. These included the subcutaneous injection of various coloured marker substances by hypodermic needle and by jet inoculator, the bulk immersion of animals in dyes and the clipping of fins. Mercuric sulphide and colored liquid latex, introduced subcutaneously by hypodermic injection, were found to be the most successful methods, and various colours of liquid latex (Diptex 319) (which had several advantages over mercuric sulphide) were chosen for use in the field.