This paper reports the results of experiments on the toxicity of alkaline to fishesin 1980--1981. The effects of different composition of alkaline as well as their internalrelationship in causing toxicities are discussed.It is noticed that alkaline (A) and pH are interdependent in producing the toxica-tion on fishes. The relationship of the 24h TLm is pH = (10.00+0.038)-(0.0149+0.007) A(n = 25,r= -0.976, s=0.101). Under the experimental condition, CO₃^- 24hTLm to silver carp(Hypophthalmichthys molitrix) is 12.4me/L. Besides CO₃^-,the majormortality factor, OH^-, CO₂^- and salinity also participate in synergism on different pHintervals. Therefore, alkalinity 10me/L may be taken as the danger index for waterquality in culturing silver carp and bighead carp (Aristichthys nobilis).