We compared marking success for larval and juvenile fishes immersed in alizarin red S (ARS) to that of fish immersed in alizarin complexone (AC). Postlarval and juvenile central stonerollers Campostoma anomalum and southern redbelly dace Phoxinus erythrogaster and larval white suckers Catostomus commersoni were immersed in ARS or AC at concentrations of 50-400 mg/L for 6-24 h. Both chemicals imparted a violetred mark, visible in the whole otoliths under a brightfield light source. Immersion in ARS at 200-300 mg/L for 12 or 24 h resulted in optimal marking success (100%) and minimal mortalities (0-3%). Marking success and mortalities were similar for ARS and AC in comparable treatments. Alizarin red S marks were retained for at least 160 d.