The effects of intraperitoneally implanted dummy acoustic transmitters on juvenile Atlantic salmon Salmo salar of comparable size to wild presmolts and strolls were examined in two laboratory experiments. In the first experiment, different-sized tags (24, 28, and 32 mm x 8-mm diameter) that mimicked acoustic transmitters suitable for long-term studies were assessed. There was 60% mortality (3 of 5 fish) with the 32-mm implant early in the 316-d study and 20% mortality (1 of 5 fish) with the 28-mm implant. All fish surviving with 28- and 32-mm implants eventually expelled their tags, compared with only 20% of fish (1 of 5) with the 24-mm implant. Tag expulsion was not a cause of death. After implantation, tags were first encapsulated in a thick membrane and then expelled between 142 and 217 d after surgery, exiting at a site where pressure of the tag on the abdominal wall was greatest. Growth of tagged fish was initially delayed and lagged that of control and sham-tagged fish, but survivors from the tagged groups eventually caught up and grew at the same rate as the other two groups. In the second experiment, 28-mm implants, representing 8.5% of body weight, had a negative effect on the fish 1 and 3 d after surgery by slightly reducing critical swimming speed compared with that of control fish. However, the swimming performance of tagged fish began to recover after 7 d. Surgical implantation of acoustic transmitters can meet the requirements for tagging and tracking small Atlantic salmon over a period of months. However, we recommend a transmitter length of 16% or less of fish length and a transmitter weight under 8% of fish weight for juvenile Atlantic salmon of about 14-15 cm and 35-45 g in order to maximize the chances of success in telemetry studies. [References: 21]