The initial goal of the Controlled Ecological Life Support System (CELSS) Breadboard Project is to develop and evaluate a ground-based bioregenerative system scaled to support the equivalent of one crew member. The Biomass Production Chamber (BPC) is the plant growing module of this project. We describe here the characterization of the microbial constituents of the BPC during production tests of hydroponically-grown crops of wheat and soybeans. Bacterial and fungal viable counts were determined for the hydroponic solution, dehumidifier condensate water, and atmosphere. Bacterial communities were characterized by taxonomic identification (Vitek AutoMicrobic System) of randomly selected isolates. For all crop tests, bacteria dominated the microflora of both the hydroponic solution (range--104 to 106 colony forming units [cfu] per mL), and dehumidifier condensate (103 to 106 cfu/mL). Occasional checks for total and fecal coliforms, Staphylococcus, Streptococcus, and Legionella have consistently proven negative. Bacterial community composition studies have focused primarily on the root surface because over 95% of the BPC microbial load occurred in this habitat. The number of root surface isolates identified and predominant bacterial genera were: Wheat--540 isolates, Pseudomonas: soybean--1080 isolates, Achromobacter. The condensate isolates identified were similar for both crops--650 isolates, with Pseudomonas pickettii predominant. In-line filtration of the BPC atmosphere has resulted in very low microbial counts (ca. 3.3 to 330 cfu/m3).