The effects of motility and aggregation on the diffusion coefficient for bacteria were studied in an aqueous system. The effects of cell concentrations, capillary tube sizes, and dilution rates on the diffusion coefficient were ex-amined. In general, motile cells can diffuse about 1000 times faster than non-motile cells. Pseudomonas aeruginosa, a motile cell, and Klebsiella pneumoniae, a non-motile cell, were used for this research. Diffusion coefficients were meas-ured by the capillary tube assay developed by Adler[1969]. From this procedure the diffusion coefficient of Pseudomonas aeruginosa was 2.1×10^-5 (standard deviation:1.0×1.0^-5)cm²/s and that of Klebsiella pneumoniae was 0.9×10^-5(standard deviation:0.5×10^-5)cm²/s. The diffusion coefficient of Pseudomonas aeruginosa was about 2.3 times higher than that of Klebsiella pneumoniae. The Stokes-Einstein equation could not be sued for estimating the diffusion coef-ficients for Klebsiella pneumoniae and Pseudomonas aeruginosa. The experimental value for the diffusion coefficient of Klebsiella pneumoniae was about 2000 times higher than that(4.5×10^-9cm²/s) obtained from the Stokes-Einstein equation. This discrepancy was due to the aggregation of Klebsiella pneumoniae.