The results are given of a study of Trichodinid infection of speckled trout. In the absence of cirri the present parasite is assigned to the genus Trichodina. A brief description of the form is given. Experiments were undertaken to produce a resistant stage for the transmission of this parasite, but without result. Simple transmission is shown to develop a heavy infection rapidly, and dead or dying parasitized fish are demonstrated to be a possible source of contamination for a great length of time—10 to 140 hours, depending upon the temperature. In the absence of a resistant stage, the drying of troughs and instruments is a satisfactory sterilization of equipment. As a prophylactic measure, a salt bath using a 3 per cent solution for two minutes (as commonly recommended) gives complete elimination of the parasite, but only when a volume of at least 50 cubic centimeters of solution is used for each fish of fingerling size. Salt solution becomes inactive very rapidly, an effect not to be considered as the result of dilution. Glacial acetic acid (1:1,500 for fifteen seconds) is a preferable solution on this account and is more positive in action. Proper treatment with these solutions can completely eliminate the parasite and bring about rapid healing of damaged tissues.