Fiber from unopened, nearly-mature field-grown bolls without punctures through the carpellary walls was sterile. Fiber from partially opened bolls in which poorly fluffed and discolored "microbial tight locks" had developed frequently had total bacterial numbers of 10⁸ to 10¹⁰ per gram; i.e. much above the 10⁶ level in bulk commercial fiber. Fiber which had fluffed well and then been subjected to 0-10 weeks of humid field weathering exhibited increasing total bacterial numbers with time of weathering, but bolls subjected to dry-climate weathering at Shafter, CA, maintained low bacterial levels. A similar field-weathering test of 4 cotton genotypes at College Station, TX, showed increasing bacterial numbers but no differences among the genotypes. Fiber stored as seed-cotton in two non-self-heating modules exhibited bacterial losses in the first 3 weeks of storage, with variable further decreases in the next 6 weeks. Fiber subjected to 0, 2, 4, and 6 passes through a standard lint cleaner showed no certain decreases in total bacterial content; the foreign matter removed had about 10 times the total bacterial concentration of the lint. Graded commer cial samples of fiber from the 1981 crop across the Cotton Belt yielded bacterial numbers similar to those reported for the 1980 crop.