Abstract: The positive correlation exists between the quantity of Cryptomonas ovata and the con-centration of NH₄⁺ in fish pond.Cryptomonas ovata begins to reproduce and increase quantitatively under the conditions of the pond water in which the temperature is 15℃ or so;the concentration of NH₄⁺ reaches up to 0.5—0.7 mg/l;NO₃^- not lower than 0.04rag/l;P₂O₅ not lower than 0.15mg/l and there is a trace of NO₂^-,Cryptomonas ovata can notproliferate if there is not adequate NH₄⁺ even though there is a considerable quantity of NO₃^-,NO₂^- and P₂O₅.