AbstractTreflan (trifluralin, Elanco) is among the compounds used in the treatment of larval mycosis in penaeid shrimp caused by the phycomycetous fungihagenidiumsp. andSirolpidiumsp. Some culturists have reported treatments for these fungi with trifluralin to be ineffective while others have found it to be quite efficacious.To study losses of trifluralin in seawater systems, experiments were conducted under varying conditions of aeration, lighting, and algal biomass using nominal concentrations of 19.3 to 65.5 ppb trifluralin. There was no detectable loss of trifluralin over a six hour period in seawater mechanically stirred and covered to prevent photodegradation. Under varying conditions of aeration and light, the estimated half‐life of trifluralin ranged from 30 to 138 min. When diatoms were added to the seawater, trifluralin levels dropped to 4% of theoretical within two to three min. These observations confirm the need for a continuous administration for trifluralin to be effective in treating larval mycoses in penaeid shrimp hatching facilitie