Bacteria dislodged from raw cotton fiber into mill air during mechanical processing have been suspected by some, investigators to be a cause of byssinosis in millworkers. This has led us to an investigation of bacterial levels on U.S. produced cotton. Data from the U.S. cotton crops of 1980, 1981, and 1982 showed that many cotton samples from most U.S. growing areas had high bacterial counts but that samples from the San Joaquin Valley of California were much lower in bacterial counts than the average for samples from other areas. These low counts have been attributed by us to the low level of rainfall which usually occurs in the Valley during the period from boll opening to harvest. Bacterial counts for 1983-crop samples from California again exhibited low counts on fiber from the San Joaquin Valley.