Distinguishing hatchery fish from wild is a fundamental part of stock enhancement programs. Batch marking with chemicals, such as oxytetracycline, is common. Calcein offers and advantage over oxytetracycline because of the non-lethal nature of mark detection. However, ultraviolet light is reported to degrade external calcein marks, so mark retention in full sunlight should be examined. We attempted to determine the retention of calcein marks on fingerling (< 1 g) Largemouth Bass Micropterus salmoides. This work was conducted under an agreement with the U.S. Fish and Wildlife Service (INAD #10-987). Prior work with crappies suggested osmotic induction with a 4% salt bath for 3.5 min followed by a 0.5% calcein solution for 7 min produced lasting external marks on another centrarchid. However, this procedure resulted in 100% mortality within 48 h of marking. Fish were stocked into outdoor tanks in this and every subsequent trial unless otherwise specified. A second trial was conducted with graded levels of salt (0%, 2%, and 4%) followed by a 7-min bath in a 0.5% calcein solution. Calcein solutions in this and all subsequently-described trials were always 0.5%. Mortality (48-h) was 100% in all treatment except the control, which experienced no mortality. A third trial with graded levels of salt (0%, 2%, and 4%) and a sham 7-min bath (no calcein) resulted in no mortalities after 48 h. A fourth trial with 2% salt and graded periods (3, 5, and 7 min) of 0.5% calcein bath resulted in 80% mortality after 48 h in all treatments and no mortalities in the control. We speculated that our calcein was contaminated and ordered fresh calcein. A fifth trial was conducted with 2% salt and 3.5-min bath in 5% calcein or 3.5-min in a sham bath. These fish were held in indoor tanks for 48 h and mortality in the treatment and control were both zero. A sixth trial was conducted with 2% salt and graded periods (3, 5, and 7 min) of 0.5% calcein bath or a 7-min sham. Fish were stocked directly outside. All treatments experienced 100% mortality, while the control experience none. A seventh trial was conducted with 2% salt and a 3-min 0.5% calcein bath or 3-min sham. Half the treatment and control replicates were stocked directly outside, while half the treatment and control replicates were stocked inside. All treatments and replicates experienced no mortalities after 48 h, except the treatments stocked directly outside, which experienced 100% mortality. A final trial was conducted with graded levels of salt (0%, 2%, and 4%) and a 3-min bath in 0.5% calcein. The control for this trial was 0% salt and 3-min sham calcein bath. All fish were held inside for 48 h. No mortalities occurred in any treatment except the 0% salt which experienced 10% mortality. After 48 h, fish were moved to outside tanks. Within 48 h, mortalities ranged from 60% to 80% in treatments, while the control experienced 10% mortality. A protocol for calcein marking of fingerling Largemouth Bass has not been established.