The effects of tagging with acoustic transmitters on the growth of juvenile seabass, Dicentrarchus labrax (L.) (initial mean mass +/- SD, 173 g +/- 23.4) in a 47 d tank experiment, and sole. Solea solea (L.) ( 103.2 g +/- 14.8) in a 72 d tank experiment and (104.0 g +/- 18.4) in a 58 d salt marsh mesocosm experiment were examined. Twenty externally tagged seabass grew more slowly than the 20 with surgically implanted tags, which reached the same mass as nine control fish. Movements of the external transmitter's harness caused abrasions of the skin and loss of the tag in 60% of the cases. We thus recommend implanting transmitters for telemetry studies of juvenile seabass weighing between 120 and 214 g and carrying a tag that represents 2.2-2.5% of body mass. Both tank and mesocosm experiments conducted on juvenile sole concluded that the externally attached tag retention rate was good, but at the expense of the fish growth rate.