Bacterial concentrations and percentages of gram-negative bacteria were determined in raw cotton fiber samples collected from 21 cultivars and lines of cotton grown in the Brazos River valley near College Station, Texas, in 1985. The cultivars and lines varied in levels of resistance to bacterial blight and in occurrence of mutant morphological characters (glandless, nectariless, frego-bract, okra-leaf, smooth stem, and stormproof boll) known to affect incidences of insect damage and boll rots. The analysis of variance indicated significant variations in numbers of bacteria per gram of fiber among cultivars, and highly significant differences among samples. Although up to five-fold differences occurred in bacterial numbers among cultivars, these were not related to levels of blight resistance and could only be partially explained by morphological characters. Lower bacterial numbers occurred on cottons with okra leaf or frego bract characters and higher numbers occurred on those with smooth stem, glandless, nectariless and stormproof boll characters compared to appropriate normal cultivars. However, except for smooth stem and stormproof boll, the differences were not statistically significant (P = 0.05) with the sample numbers used. Eighty-three to 98% of the bacteria in samples were gram-negative with no significant differences among cultivars. The nine samples used per cultivar in these experiments were inadequate to clearly distinguish differences among cultivars; we show that 40 or more samples per cultivar appear to be required to determine significant differences in bacterial populations on cotton fiber.