Recent advances in PIT technology have led to smaller tags, meaning that this technology can be used to mark and monitor smaller-bodied species, many of which are threatened. We examined the effects of 9-mm PIT tags on the survival and growth of the Mountain Galaxias Galaxias olidus, using this fish as a surrogate for several threatened, small-bodied galaxiids. We measured survival, growth, and tag retention in 34 tagged and 34 untagged fish held in aquaria for 90 d posttagging. Fish were randomly assigned to a treatment (tagged or untagged) and were weighed just prior to tagging and then at 7, 14, 21, 28, 42, and 90 d after tagging, with observations made 5–7 times per week to check for survival and tag loss. Survival did not differ significantly between tagged and untagged groups (79% and 88%, respectively), and there was no difference in weight gain or loss between the two groups over time. Tag retention rate was high (96%), with only one tag expelled by the smallest tagged fish (73 mm length to caudal fork). Our findings show that Mountain Galaxias are capable of successfully retaining 9-mm PIT tags in aquarium conditions, suggesting that this technology is suitable for the monitoring of similar small-bodied fish in the wild.