The purpose of the research is to determine safe parameters for the use of certain fatty acid amides against protozoal diseases of fish.Materials and methods. The synthesis and biological activity determination of new compounds were studied in laboratory conditions of the chemical synthesis creative team of the North Caucasus Zonal Research Veterinary Institute. The protistocidal activity of the new substances was determined in a liquid nutritional medium by serial dilution according to our technique. The ciliate Colpoda steinii was used as a test culture. The solution toxicity was determined on aquarium Guppy (Poecilia reticulata). The efficacy was assessed according to an alternative form of the response: the minimum concentration that all protozoan species died in was considered as protistocidal; the maximum concentration that all fish remained alive in was recognized as safe for the fish, i.e., the first concentration after the absolute lethal concentrations. Compliance with these conditions required the prepared solutions having minimal concentration ranges from 1 to 0.1 µg/mL, as well as continuous monitoring with recording the results. Two compounds were tested, namely, myristic acid amide and oleic acid amide.Results and discussion. The fish remained viable in the aqueous myristic acid amide solution concentration of 0.2 μg/mL for 3 days (monitoring period), and the fish remained alive for 3.5 hours or more (up to 20 hours) in the solution concentration of 0.5 μg/mL. At the same time, a concentration of 0.14 μg/mL had a protistocidal effect on the protozoan C. steinii if exposed for 40 minutes. The maximum tolerated oleic acid amide concentration for fish was 1 µg/mL if exposed for 2 or more days and 2 µg/mL if exposed for 4 h. The minimum protistocidal activity level of oleic acid amide was 0.58 µg/mL at an 18 h exposure.