Spacecraft assembly facilities (SAFs) house clean rooms where interplanetary spacecraft are built, thereby reducing the bioburden on spacecraft to protect planetary environments from terrestrial microbes that may interfere with the search for life or disturb potential native ecosystems. The most plausible environments for living systems on celestial bodies involve brines with depressed freezing points. Here, we specifically measure the abundance of salinotolerant microbes on SAF surfaces. Most probable number analyses performed with salty liquid media were applied to washes of SAF floor wipes. Microbial abundance was measured using Salt Plains medium at low salt or supplemented with (all w/v) 10% NaCl (1.7 M; aw = 0.92), 50% MgSO4 (2.0 M as epsomite; aw = 0.94), 5% NaClO3 (0.5 M; aw = 0.98), or 5% NaClO4 (0.4 M; aw = 0.98). The abundance of salinotolerant microbes was generally 1 to 10% (102 to 104 cells m?2) of the total population of microbes observed in low-salt medium (105 cells m?2). Microbes were isolated by repetitive streak-plating of positive enrichment cultures and then characterized. All of the 38 isolates were Gram-positive bacteria, mainly spore-forming Bacillaceae, with some Staphylococcus. The isolate collection showed strong tolerance to high concentrations of NaCl (to 30%), MgSO4 (to 50%) and sucrose (to 70%). There also was substantial tolerance to pH (5 to 10) and temperature (4 to 60 °C). Taken together, these SAF isolates are polyextremophiles that are in substantial abundance in the clean rooms where spacecraft are assembled.