The phonological submodel of the PUTU wheat crop growth model was refined to account for the variation in sensitivity to vernalization and photoperiod. Crop development rate was simulated in five phenological phases making use of temperature and photoperiod. Base values of temperature, photoperiod and vernalization were determined for 22 cultivars currently grown in the RSA. After calibration the submodel was verified for two cultivars. It was shown on average to calculate accurately the date of flag leaf maturity and start of anthesis to within six and five days, respectively, of the true date. It was concluded that the phenological submodel of PUTU wheat crop growth model functioned satisfactorily.