Jackson (1959) introduced a technique for mass‐marking fish that entails forcing fluorescent pigment into the dermal tissue with compressed air from a small sandblast gun. The technique is faster and less expensive than existing marking methods and is particularly useful in short‐term studies of the dynamics and distribution of fish populations. Several experiments were conducted to define the best pigment size and air pressure for marking young salmonids, viz., pink, coho and sockeye salmon (Oncorhynchus gorbuscha, O. kisutch, and O. nerka) and rainbow trout (Salmo gairdneri), ranging in length from 30 to 120 mm. Two pigment sizes were used: powder, which was less than 20 µ in diameter, and granular pigment, which varied from 30 to 350 µ in diameter. The granular pigment was retained significantly better (55.0–100.0%) than the powder (6.1–89.0%) in test periods ranging up to 130 days. Mark retention among the experimental groups was comparable at air pressures between 80 and 160 psi. Mortality directly attributable to marking was negligible.