A study was undertaken of the relative sampling efficiencies of eight bioaerosol samplers. Each was individually challenged with a bioaerosol of either Bacillus-subtilis or Escherichia-coli. The samplers were evaluated under controlled conditions in a horizontal bioaerosol chamber. During each run simultaneous samples were collected with a reference Ace Glass All Glass Impinger-30 (AGI-30) to verify the concentration of microorganisms in the chamber from run to run and day to day. The collection efficiency of the evaluated AGI-30 relative to the reference AGI-30 was 100%. The Andersen Six Stage Viable Particle Sizing Sampler (6-STG) over sampled the reference AGI-30 by about 7%. However, the Andersen Two Stage samplers undersampled the reference AGI-30 by 8%, 11%, and 32%, respectively. The relative collection efficiencies of the Gelman 47mm Membrane Filter, PBI Surface Air System and Biotest Reuter Centrifugal Sampler were less than 1% for E-coli. The low relative efficiency of the Membrane Filter with E-coli was probably due to desiccation of the organism. The Surface Air System and the Reuter Centrifugal Sampler were not efficient collectors of small particles. The relative efficiency of the Membrane Filter with B-subtilis was similar to that of the reference AGI-30. For aerosols of free bacteria, the Andersen Six Stage impactor, the Ace Glass AGI-30, and the Andersen One Stage impactor gave comparable results.