Indian almond (Terminalia catappa Linn.) leaves with green and red coloration were tested for bactericidal activity with pathogenic bacteria and their acute toxicity to Siamese fighting fish (Betta splendens Regan). Powder of Indian almond leaves was extracted with water at the ratio of 1:10 (w/v) and then freeze-dried to a dry powder. Bactericidal efficacy was tested against 28 isolates of pathogenic bacteria (Aeromonas hydrophila, Edwardsiella tarda, Enterobacter spp., Plesiomonas shigelloides, Pseudomonas spp., Shewanella putrefaciens, Staphylococcus sp. and Streptococcus sp.) isolated from Siamese fighting fish. A paper disc diffusion method was tested on Mueller Hinton Agar (MHA) by inoculating 1 x 106 CFU/mL of each bacterial suspension. Paper disks (5 mm) were impregnated with either 10 microl of green or red Indian almond leaf extract at a concentration of 12,000 ppm, then laid on the surface of the MHA. The results revealed that aqueous extract of red Indian almond leaves could inhibit the growth of tested bacteria better than the green extract. The minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) were determined. Extracts of green and red leaves were diluted in Mueller Hinton Broth (MHB) to obtain a starting extract concentration of 6,000 ppm then twofold serially diluted in a 96-well microtitre plate. The pathogenic bacteria were inoculated into each well at a density of 1 x 105 CFU/mL and incubated at 35 degrees C for 24 h. The growth of bacteria was detected by 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazoliium bromide. The MIC of the red leaf extract ranged from 187.5 to 375 ppm which was lower than for green leaf extract (375-750 ppm). The MBC range of the red leaf extract was 375-750 ppm which was lower than for the green leaf extract (750-1,500 ppm). Acute toxicity tests (96-h LC50) of Indian almond green and red leaf water extracts in Siamese fighting fish were conducted. A logarithmic-spaced series of Indian almond leaf extract concentrations (6 concentrations) between the highest concentration that did not kill fish and the lowest concentration that killed all fish were used. Each concentration had three replicates (20 fish/replication). Mortality was observed after 96 hours. The LC50 value was calculated using probit analysis. The 96-h LC50 value for green and red leaf extracts was 1,765.69 and 1,651.21 ppm, respectively. When Siamese fighting fish were cultured in water added with Indian almond water extract at 5 concentrations for 5 days and then challenged with Aeromonas hydrophila at a concentration of 1 x 106 CFU/mL, the survival rate of the Siamese fighting fish infected by A. hydrophila could be decreased by bathing with Indian almond red leaf extract at 750 ppm while green leaf Indian almond extract was effective for Aeromonad septicemia therapy in Siamese fighting fish when used at a concentration of 1,000 ppm. In conclusion, red leaf Indian almond aqueous extract had high potential for the control of pathogenic bacteria at a concentration of 750 ppm which should be safe for Siamese fighting fish taking into consideration the toxic level of the extract.