The effects of temperature and of ultraviolet radiation on the multiplication of bacteriophage phi29 were studied. Samples of phi29 that had been irradiated to surviving fractions of 0.44 or 0.10 were propagated at 37 degrees C, 42 degrees C and 43.5 degrees C. Latent periods and burst sizes were obtained from one-step growth curves. At a particular temperature, as the dose delivered to the virus was increased, the latent period was extended and the burst size was decreased. For unirradiated virus, the burst size was the same at 42 degrees C as at 37 degrees C, but decreased dramatically at 43.5 degrees C. For virus subjected to a particular dose, the burst size decreased as the temperature was raised. A statistical technique for improving the reliability of parameters obtained from one-step growth curves is presented.