Dependence of acoustic size (target strength-TS in dB) on body length of juvenile perch (Perca fluviatilis L.) was studied. Perch larvae were reared in a laboratory tank from spawning to the age of 89 days. Fish length and TS were measured during 3, 7, 15, 22, 45 and 89 days of life. TS was measured using a Simrad 500 EY split-beam echosounder with the frequency 120 kHz and a BioSonics Model 101 dual-beam echosounder with the frequency 420 kHz in down-looking, horizontal and up-looking directions. Range of fish body length 7-41 mm corresponded with a TS range from -69 to -53 dB. Differences between TS of different aspects were not found in young fish larvae. TS in ventral and dorsal aspects appears to grow faster than in the side aspect for fish 15-30 mm long. With perch larger than 40 mm, vertical and side aspects reach similar values. This paper provides regressions for the size identification of acoustically detected fish targets. The results indicate that the TS/length relationship of early life stages of perch is not too different from general fish TS/length relationship. (C) 2003 Elsevier B.V. All Rights Reserved.