Trichlorophon is one of the organophosphate poisons that are used with different objectives in aquaculture. The current study dealt with the measurement of acute toxicity caused by Trichlorophon on six species of ornamental fish, namely the red sword tail, Tiger, Barbie, bright tetra, guppies, Green-fin molly and Gambusia. To this end, all species were exposed to 5 treatments (with three replicates for each treatment) under different concentrations of toxin containing 0, 5, 10, 20 and 40 ppm They were treated for 96 hours and the rate of mortality was recorded. Toxicity tests were determined using probit analysis in the statistical software SPSS (version 16). At the end of the experiment, the LC50 96 hours of Trichlorophon in the red sword tail, Tiger, Barbie, bright tetra, guppies, Green-fin molly and Gambusia were 7.99, 8.74, 8.88, 9.80, 9.80 and 9.95 mg/liter, respectively. Eventually, Trichlorophon was classified as partly toxic for ornamental fish causingvery similar lethal ranges for all six species, though the red sword tail fish was more sensitive to this toxin.