Graduation date: 2007Effective treatment of larval bacterial diseases is a difficult problem when culturing ornamental fish. Oral administration of antibiotics using existing microparticle types is not effective due to high leakage rates; furthermore, injection of larvae is not practical. Treatment is currently limited to use of antibiotic baths, . In this study, we evaluated lipid spray beads (LSB) for delivery of the low-molecular weight, water-soluble antibiotic, oxytetracycline·HCl (OTC) to fish larvae. Lipid spray beads composed of menhaden stearine have shown good retention of water-soluble core materials and high acceptability by larval fish. Various OTC core-to-lipid ratios and OTC core concentrations were evaluated to maximize OTC delivery efficiencies. Acceptability and digestion of LSB containing riboflavin by larval zebrafish, Danio rerio (Hamilton), and larval gobies, Asterropteryx semipunctata (Rüppell), were also evaluated. Increasing LSB core-to-lipid ratios from 1:3 v/v to 1:1 v/v resulted in an increase of encapsulation efficiency from 2.33% w/w to 3.68% w/w. LSB prepared with a ratio of 1:1.25 v/v were then used to determine the effect of core concentration on encapsulation, retention and delivery efficiencies. Increasing concentrations of OTC in the core from 0.1 to 0.5 g OTC mL-1 H2O caused encapsulation efficiencies to increase from 3.95% w/w to 18.77% w/w, respectively. This increase did not affect retention efficiencies and improved delivery efficiencies. Lipid spray beads prepared with a core concentration of 0.5 g OTC mL-1 H2O produced the highest delivery efficiencies (7.9 ± 0.7% w/w) after suspending the beads in water for 60 min60 min aqueous suspension Consumption of LSB containing OTC by first-feeding zebrafish and goby larvae was confirmed by analysis of feeding incidence and gut fullness. Visual observations of larvae fed on LSB containing riboflavin indicated that LSB were digested by larvae of both species. Particles were compacted and brightly fluorescent, freely-dissolved riboflavin filled the gut lumen and exited the anus in plumes. These results suggested that LSB composed of menhaden stearine could be used to deliver OTC to larval fish and may be useful in treating larval fish infected with bacteria susceptible to OTC. Oxytetracycline dosage could be controlled by either varying the concentration of administered LSB or the concentration of core solution.