A pair of putatively isogenic pss(Ts) and pss+ (phosphatidylserine synthetase structural gene) strains was constructed and analyzed, together with the revertants, for the physiological consequences of cessation of the optimal synthesis of phosphatidylethanolamine (PE). Their in vivo and in vitro abilities to synthetize PE and the growth rates at different temperatures were determined. The rate of PE synthesis by OS2101 pss(Ts) was inversely related to the culture temperature. OS2101 in a low-salt broth medium stopped division and formed filamentous cells with declining viability upon the elevation of culture temperature from 27 to 42 or 44 degrees C, whereas the syntheses of deoxyribonucleic acid, ribonucleic acid, and protein were not affected. Proper concentrations of cations such as Na+, K+, NH4+, and Mg2+ or of sucrose could remedy the division and growth of OS2101 at the restrictive temperature without restoring normal PE synthesis. A remedial effect other than osmotic protection of these effectors and an adaptive regulatory mechanism for PE formation are suggested.