Settled and respirable grain dust samples were collected at intervals over a 16-month period at two grain terminals on the lower Mississippi River. Total and gram-negative aerobic bacterial (GNB) populations, as well as endotoxin concentrations, were studied. Plate counts indicated that no viable bacteria were found to be present in the respirable samples. Endotoxin concentrations ranged from 0.0 to 6.4 ng (0.0 to 7.9 ng/m3 air) per respirable sample, indicating that bacteria were once viable in dust of respirable size or contamination occurred from larger, non-respirable particles. Numbers of bacteria from settled dusts, both total and GNB, remained relatively stable; however, the genera of the GNB populations varied. Enterobacter agglomerans was the predominant species in warm months, but not in winter. Pseudomonas and Klebsiella species increased in winter and decreased in summer. Other genera found were Citrobacter and Serratia. Endotoxin concentrations in settled dust samples ranged from 1.7 to 5.6 ng/mg dust.