Quantitative data were collected to describe the relation between temperature and growth of the cabbage black rot pathogen,Xanthomonas campestris pv.campestris (Xcc). Relative growth rates derived from experiments at constant temperatures were used in dynamic simulation of bacterial population development. The relative growth rates were adequate to simulate growth ofXcc populations at constant temperatures but overestimated growth of populations at variable temperatures. This finding gives rise to the hypothesis, that under field conditions, disease development is slower than is expected on the basis of growth parameters obtained from studies with constant temperatures.