Hatcheries release millions of juvenile trout into rivers and lakes annually with little or no post release monitoring. This is, in part, because marking of hatchery-reared trout is often not compulsory and currently available marking methods are either considered costly, cause high mortality, inconvenient to apply, or have poor long-term retention rates. Here, we test a stable isotope larval immersion marking technique to determine if a suitable low cost, easy to apply fish friendly marking method could be achieved for brown trout Salmo trutta and rainbow trout Oncorhynchus mykiss. Larval immersion using 100 µg. L-1 of 137Ba and 136Ba over a 24 hr period returned a 100% mark-success rate for both brown and rainbow trout, at a marking cost of approximately $US 0.004 per fish. Barium isotope ratios detected in the otoliths of marked fish were 11 times greater than ratios measured in control fish. The process of marking was easy to apply, with < 0.5% mortality during the marking process. We conclude that larval immersion marking is a suitable method for long-term monitoring of released hatchery-reared trout. If adopted, the method would enable hatcheries to cost effectively and accurately assess the long-term contribution of restocked fish to wild populations