A temperature-sensitive strain (HD 20) of Salmonella typhimurium is described. At restrictive temperature this strain shows an envelope alteration and a defect in division associated with an increase in cell diameter. On a shift to 42 degrees C there is residual division for ca. 30 min and then no further increase in cell number. In minimal medium (MM) at 42 degrees C cell diameter remains unchanged for about one mass doubling and then increases. From measurements of cell elongation, it is concluded that such increases in diameter occur because cell volume increases exponentially at 42 degrees C but increases in the rate of elongation occur for only a short period and then elongation rate becomes constant. This conclusion is supported by the observation that HD 20 cells show no increase in diameter at 42 degrees C when cultured in media supplemented with sucrose and Mg++; normal increases in elongation rate occur in such media. A model which fits the experimental data has been constructed. This model has two main features namely (1) the elongation of individual cells is linear with the rate of elongation doubling close to division and (2) such doublings in elongation rate are linked to division such that division cannot occur if elongation rate has not doubled shortly before. In the mutant it is proposed that only a few doublings in elongation rate occur at 42 degrees C and these are responsible for the residual division. The model fits the data for cultures shifted to 42 degrees C in MM or in yeast extract casamino acids MM (YE Cas MM) and for cultures shifted to 42 degrees C and shifted up from MM to YE Cas MM. The observations on these medium shifted cultures suggest that the activity of growth zones responsible for elongation is medium dependent.