Microviscosity in micellar solutions of N-dodecanoyl-, N-tetradecanoyl-, and N-hexadecanoyl-DL-valinates was determined as a function of NaCl concentration with tyro fluorescence probes (auramine and 1,3-dipyrenylpropane). The microviscosity increased with increasing concentration of NaCl, its increasing trend became more steep above a threshold NaCl concentration at which micellar growth began (the first break point), and then the microviscosity reached a constant value at the NaCl concentration above which the micelles interacted strongly with each other (the second break point). While micelle aggregation number increased steeply at the first break point, the solution became very viscous above the second break point. The double logarithmic plot of the critical micelle concentration against the NaCl concentration deviated downward from a linear relation above the NaCl concentration corresponding to the first break point. Each micellar solution of N-acylvalinate was divided to four regions by using the above data.