Bioreactors hold great promise for treating graywater in an advanced life support system for space applications. However, questions remain regarding the reproducibility and reliability of biological systems for long-term use. Although there have been numerous studies on ground-based biological systems, most studies focus on a single reactor or a simple (single carbon) waste stream. There have been very few studies on microbial communities in replicate reactors using a nonsterile, complex waste stream. In this report, we describe the characterization of five replicate denitrifying reactors receiving a complex feed, including urine and limb washes from donors at Johnson Space Center over a 100-day period. Denitrifying conditions were employed because of the ease in adding a terminal electron acceptor to the bioreactor. Bacterial populations were tracked by 16S rRNA and nosZ genes T-RFLP analysis to target the total and denitrifying microbial communities. The results demonstrated reproducible biological communities with nearly identical performance that slowly changed with time and exhibited low variability with respect to the bacterial community (T-RFLP peak area) in all reactors. These results suggest that, when designed for replication, bioreactors are not stochastic systems exhibiting chaotic behavior, but are biological systems that can be highly reproducible and reliable.