The need of aquarists for proper and safe disinfection of fish with Chloramine-T (CL-T) necessitates toxico/pathologic examinations. This study assays toxicity level of CL-T and histopathologic changes of gills, liver, kidney and brain of treated zebrafish (Danio rerio). Groups of 10 Zebrafish were treated with 0 (control), 5, 10, 15, 30, 50,100 and 200 mg/L of CL– T. Mortality was recorded at every 6 hours until 96h after the beginning of the treatment and lethal concentration for 50% of population (LC50 at 24h and 96h) was calculated. The histopathologic samples were taken from dying fish during the exposure period. Histopathological changes were not evident in organs of the control group. In the gills of treatment groups, congestion, edema, separation of epithelium of secondary lamella from basal membrane, fusion and hypertrophy of secondary lamella, telangiectasis and necrosis of lamella epithelium were observed. In the liver of treatment groups degenerative changes and necrosis of hepatocytes were visible. The kidney samples represented hydropic degeneration, necrosis, dilation of periglomerular urinary space and hyaline droplets in renal tubules in high concentration. The brain of treatment groups showed spongiosis and increase in glial cells. Severity of these lesions was related to the concentration of CL-T. LC50 24h for CL-T was 428.649 mg/L and LC50 96h was 11.044 mg/L which reveals the low level of toxicity for few hours of treatment. The concentrations below 15mg/L are reasonably safe and recommended for usage in D. rerio treatment during 24h of bath.