We have examined the nonlinear behavior of the fluoroperovskites CsCdF3 and KZnF3 which are known to exhibit similar linear elastic behavior. The temperature variation of some combinations of the third-order elastic (TOE) constants have been measured as a function of temperature using the ultrasonic harmonic generation technique. Measurements have been made from room temperature to 77°K. It is observed that the constant C111 shows a higher temperature variation than it does in other cubic crystals such as copper or silicon. Between 300° and 77°C, C111 changes by 8.5% for CsCdF3 and 23.5% for KZnF3. The combination (C112 + 4C166) does not vary much with temperature, whereas the combination (C123 + 6C144 + 8C456) has a very high (negative) value and a peculiar temperature dependence, showing that these compounds are far away from being solids for which a central, nearest-neighbors model would be appropriate. [Research sponsored by the Office of Naval Research.]