Abstract: Spore-forming microbes recovered from spacecraft surfaces and assembly facilities were exposed to simulated martian UV irradiation. The effects of UVA (315-400 nm), UVA+B (280-400 nm), and full spectrum (200-400 nm) on the survival of microorganisms were studied at UV intensities expected to strike the surfaces of Mars. Microbial species isolated from several spacecraft surfaces such as Mars Odyssey, X-2000 [avionics], and the International Space Station and their assembly facilities were identified using 16S rDNA sequencing. Among 43 Bacillus spore lines screened, 19 isolates showed resistance to UVC irradiation after exposure to 1,000 J m?2 of UVC at 254 nm using a low-pressure mercury lamp. Spores of Bacillus species isolated from spacecraft-associated surfaces were more resistant than a standard dosimetric strain, B. subtilis 168.