Microorganisms are known to play an important role in shaping the life histories of animals. Recent studies have proposed that the coloration of birds' plumage could reflect individual quality through associations with feather-degrading bacteria. However, few studies have explored such relationships. We studied breeding female Great Tits (Parus major) during nest building and chick rearing to explore associations between bacteria inhabiting their yellow chest feathers and feather coloration. Specifically, we used flow cytometry and ribosomal intergenic spacer analysis (RISA), respectively, to study the densities of all free-living and attached bacteria and the phylotypic richness of feather-degrading bacterial assemblages. We used chroma (color saturation) as a measure of feather coloration. During chick rearing but not during nest building, the female's chroma was negatively related to the phylotypic richness of feather-degrading bacteria. Also, a seasonal change in the density of attached bacteria associating with individual birds was negatively associated with change in chroma over the same period. These findings suggest that conspicuous coloration of female Great Tits may reflect the numbers and character of bacteria inhabiting feathers.