Microgravity elicits different responses from various microorganisms and may alter the population dynamics of a microbial community. Four isolates from a hydroponic study were combined in experimental communities cultured in 50-mL high aspect-ratio rotating-wall vessel bioreactors (HARVs) for 10 days under either simulated microgravity (SMG) or control (1.004×g) conditions. Pseudomonas azelaica maintained a population density just above 1×109 CFU/mL in all treatments, but exhibited an increased percentage of a crinkled colony morphology in SMG. Abundance of Rhodotorula rubra was unaffected by either SMG or a 4-strain mixed culture environment alone; however, in a 4-strain community cultured under SMG, density of R. rubra dropped by more than two orders of magnitude. Interactive effects between SMG and presence of Pseudomonas seemed to inhibit growth of R. rubra. These interactive effects were not predictable from separate study of the components of the system. Examination of microbial interactions in microgravity is required for long-term control of hazardous microbes or maintenance of necessary microbes during extended space missions.