Changes in body size in fish can alter the pharmacokinetics of drugs, thereby affecting their therapeutic efficacy. Fish are among the animals that exhibit the greatest variability in body size during their life cycle. Despite being exposed to bacterial infections at all stages of life, the dosage regimens of antibacterial drugs have not been adjusted for age/body mass changes. The plasma and tissue pharmacokinetics of danofloxacin differed according to rainbow trout size. Pharmacokinetic/pharmacodynamic findings suggest that treatment efficacy is positively correlated with body size. However, in natural infections caused by susceptible bacteria, pharmacokinetic/pharmacodynamic studies in different fish sizes are needed to define optimal dosing regimens.