Smooth strains of Salmonella typhimurium and S. minnesota, and chemotypes Ra, Rb, and Rc, which are deficient in lipopolysaccharide components of the somatic side chains and outer core region, grow normally on nutrient agar and nutrient broth up to 45 °C. However, most mutants with defects in the heptose region of the LPS (chemotypes Rd2 and Re) do not grow on this medium at 42 °C or above; a few grow at 42 °C but not at 45 °C. In liquid medium (nutrient broth, or phosphate minimal medium), growth, measured as turbidity or as colony-forming units, stops 60 to 90 min after shift from 30 to 42 °C; DNA and protein synthesis cease at the same time. Growth does not reoccur at 42 °C; protein synthesis and growth reinitiate upon shift to 30 or 37 °C. Growth cessation does not alter cell morphology in the phase-contrast microscope. Growth of heptose-deficient strains at 42 °C in nutrient broth is restored by MgCl2 (0.5 mM), NaCl (50 mM), or sucrose (100 mM). Sensitivity to smooth-specific and rough-specific phages, and analysis of LPS composition, indicate that heptose-deficient mutants grown at temperatures from 30 to 45 °C, and in the presence or absence of high salt, do not contain heptose or O-specific sugars in their LPS.