The first two phases of the Lunar-Mars Life Support Test Project [LMLSTP] involved housing human volunteers in closed chambers that mimic future extraterrestrial life support systems. The Phase I test involved one person living for 15 days in a chamber with wheat as the primary means of air revitalization. The Phase II test involved 4 people living for 30 days in a chamber with physical/chemical air revitalization and waste water recycling. The consequences of closure on microbial ecology and the influence that microbes had on these closed environmental life support systems were determined during both tests. The air, water, and surfaces of each chamber were sampled for microbial content before, during, and after each test. The numbers of microbes on the Phase I habitation chamber surfaces increased with length of occupation. However, the numbers of microbes on the surfaces of the Phase II chamber did not increase, a finding that contrasts with past results from other closed chamber studies and from Space Shuttle missions. The types of microbes found in the air and on the surfaces from both tests were similar. The most common bacteria were Bacillus sp. and Staphylococcus sp., with Pencillium sp. and Cladosporium sp. the most common fungi. Neither test had levels or types of microbes that would be considered an occupational health threat. One explanation for the lack of microbial colonization of the chamber surfaces during the Phase II test is that the air-handling system of this chamber was designed for habitation by 8 individuals, but the test was conducted with only 4. Moreover, the relative humidity of the chambers was maintained at 95% during Phase I and at 40% during Phase II. The lower humidity may have contributed to the lesser microbial load during the Phase II test. Results from Phase I and II tests imply that human flora may ultimately dominate the ecology of a closed life support system. Nevertheless, the effect of long-term closure on the microflora cannot be fully appreciated at this time because of the relatively short evaluation periods.