Survival of Bacillus subtilis var. globigii in a simulated Martian environment was demonstrated. Previous contact with the simulated Martian soil or atmosphere reduced germination or outgrowth of unheated spores, or both. Inoculation into simulated Martian soil and then flushing with a simulated Martian atmosphere were lethal to both vegetative cells and spores. After one diurnal temperature cycle (26 to -60 C), the majority of of cells present were spores. No further effect of the diurnal cycle on survival was noted in any of the experimental samples.