The efficiency with which immersion in alizarin red S (ARS) produced marks in the otoliths of large juvenile fish was analysed. Turbots (approximately 136 mm total length and 45 g wet weight) were marked for 24 h using four treatments of seawater solutions of ARS (buffered ARS 100, 200, 400 mg l(-1) and unbuffered ARS 400 mg l(-1)). A single treatment with an unbuffered seawater solution of alizarin complexone (AC) (set to 120 mg l(-1) from preliminary trials) was used as a reference marker. All fish survived to the end of the experiment (33 days after marking), and their growth rates did not differ significantly between groups. Examination of frontal sections of otoliths under UV light microscopy revealed the yellowish and pink marks induced by ARS and AC, respectively. The quality of marks was equal for both AC and ARS immersions, at a 400 mg l(-1) concentration of ARS. ARS immersion has the double advantage of being an inexpensive and (probably) low-stress technique for marking the otoliths of large turbot juveniles. The application of ARS to large juveniles of other marine and freshwater species is suggested. (C) 2000 International Council for the Exploration of the Sea.