Centrally notched plate specimens of PMMA and PC were cycled in tension between constant stress intensity limits and crack growth was monitored against the number of cycles. A range of frequencies between 0.1 IIz and 100 Hz, and of temperatures between − 70 °C and + 40 °C was investigated. It was found that the cyclic crack growth decreased with decreasing temperature and/or increasing frequency. A linear relationship between crack length and number of cycles was observed. Application of the cyclic crack propagation law proposed byArad-Radon-Culver, namelyd(2a)/dN = βλn where λ is (Kmax2 -Kmin2) and Kmax andKmin are the respective values of maximum and minimum stress intensity factors, indicates that the indexn is unaffected by temperature or frequency variations in the investigated range (II regime of fatigue crack propagation) and the law would be a valid model if the termβ is suitably determined.