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Muth used tap water or natural freshwater to prepare tetracycline or ALC solutions at concentrations similar to ours with no reported problems. Also, otoliths in early larvae of razorback sucker (:::;; 12 days old, posthatching) reared at Ouray Fish Hatchery, Utah, were successfully marked with a TC treatment of 350 mg/l for 6.5 h using dilution water with total hardness of about 150 mg/l (R. T. Muth, pers. obser.). If water having high total hardness is used to prepare ALC or TC solutions, concentrations of these chemicals may have to be increased beyond recommended levels in distilled water for successful marking of otoliths. Tsukamoto (1988) stated that multiple treatments with one or more chemicals of different fluorescent colors can provide many variations of otolith marks for uniquely marking different batches of fish. In our two-chemical double marking of otoliths in newly hatched razorback sucker larvae, complete separation of marks was not achieved with a 24-h interval between treatments. However, presence of a double mark was obvious because of sharp contrast between the red ALC and yellow TC fluorescence. A similar observation was made by Tsukamoto (1988) for ayu (PlecoQlossus altivelis) that had been treated with ALC as embryos and again, 2-3 days later, with TC as newly hatched larvae. In multiple marking with one fluorescent chemical, complete separation between marks is necessary for accurate recognition of different marking patterns. Dabrowski and Tsukamoto (1986) marked otoliths in juveniles of peled (CoreQonus Deled) with TC at three, 6-7-day intervals and reported good separation between marks. Good separation of double TC marks in otoliths of razorback sucker larvae reared at Ouray Fish Hatchery was achieved with a 7-day interval between treatments (R. T. Muth, pers. obser.). Longevity of ALC or TC marks in otoliths of razorback sucker is unknown, but they have been reported to persist in 6