A chloride suspension in saline with 12.7 μg of dysprosium or europium, or with 127 μg samarium, or with a mixture containing 12.7 μg Dy, 12.7 μg Eu, and 254 μg Sm, was injected interperitoneally in fingerling rainbow troutOncorhynchus mykiss. The retention and tissue distribution of these rare-earth marks, and the effects of these marks on fish growth and development, were monitored for 763 d. Rare earth element concentrations in the fish were determined by neutron activation analysis. Mark retention and tissue distribution were similar in fish receiving the lanthanides singly or in combination. Soon after injection, over 90% of each element was associated with the gut. As the fish developed toward sexual maturation, small amounts of the injected elements were detected in the kidneys and gonads. After treatment, small amounts (<2%) of Sm were recovered from the liver during the first year but not thereafter. Levels of Dy and Eu declined by 40–50% during the first 6 months, but they remained stable for the next 1.5 years. Conversely, Sm levels in the fish were unchanged over the 2-year study period, even though this element became concentrated in different tissues over time. Physiological effects of marking were restricted to a transient increase in the weight of the gut relative to body weight and a slight edema in the kidney, especially in fish receiving the mixture of elements. Growth and sexual development were unaffected by the treatments.