A consistent, efficient method for marking brown trout (Salmo trutta) by intracutaneous injection of dyes was developed. Experiments were then conducted with over 150 chemicals in an effort to obtain easily identifiable, long-lasting, weightless, and harmless colored marks. Thirteen dyes in seven colors were durable. The best were National Fast Blue 8GXM, a water-soluble blue dye, and hydrated chromium oxide, an insoluble green pigment. The former most nearly meets the requirements for a satisfactory injection dye. It is non-toxic, retained on all fish for at least two years, and can be applied at a rate of 400 trout per hour. Hydrated chromium oxide was retained on all fish for one year. Encouraging results were also obtained with plastics. In field tests with pond brook trout (Salvelinus fontinalis) these two dyes were retained on all fish after one year with no adverse effects on growth and survival. A four-month test with winter flounders (Pseudopleuronectes americanus) demonstrated mark permanence for the entire period. A new high pressure jet-injection system appears promising.