Passive integrated transponder (PIT) tags have led to greater understanding of fish growth, survival, predation, migration, behavior, and the effects of human-caused environmental manipulation. Technological advances have reduced the size of PIT tags, permitting their implantation into smaller fish. To reap the benefits of PIT tag technology, implantation must not adversely impact growth, survival, or behavior. Steelhead Oncorhynchus mykiss parr reared in a natural-like environment were implanted with 12-mm PIT tags, and implantation effects on growth rate were measured. Steelhead had fork lengths ranging from 45 to 96 mm after 4 weeks of rearing in an outdoor stream channel. Thirty (17.4%) of the 172 steelhead parr that were under 74 mm at the time of PIT tag implantation had instantaneous growth rates of 0 [loge(g)]/d or less, while all parr (n = 72) over 75 mm exhibited positive growth rates. Logistic regression indicated that fork length at implantation was significantly related to the probability of positive growth rates (P < 0.001). For steelhead parr that exceeded 74 mm at implantation, the probability of experiencing positive growth was greater than 95%. The data suggest that fork length of steelhead parr should be above 74 mm at implantation to minimize negative growth effects associated with 12-mm PIT tags.