The paper examines the accelerated evolution and Muller's ratchet in endosymbiotic bacteria, specifically focusing on the genus *Buchnera*. Endosymbionts, which live within animal cells and are transmitted through cytoplasmic inheritance, exhibit distinct population structures with small population sizes and no recombination. These characteristics lead to the accumulation of mildly deleterious mutations. The study compares the evolutionary rates of 16S rDNA sequences from five independently derived endosymbiont clades to those of free-living relatives, finding significantly faster evolution in endosymbionts. For *Buchnera*, coding genes show even faster evolution and lower ratios of synonymous to nonsynonymous substitutions compared to free-living bacteria. This concentration of the rate increase at nonsynonymous sites supports the hypothesis that substitutions are deleterious in terms of polypeptide function. The observations are best explained by the combined effects of Muller's ratchet in small asexual populations and mutational bias. The loss of a repair gene and overproduction of a chaperonin in *Buchnera* may reflect compensatory evolution. An alternative hypothesis involving selection on genomic base composition is contradicted by the observed pattern of substitutions. The findings suggest that endosymbionts may undergo long-term deterioration due to mutation accumulation, potentially limiting the long-term fitness of their hosts.The paper examines the accelerated evolution and Muller's ratchet in endosymbiotic bacteria, specifically focusing on the genus *Buchnera*. Endosymbionts, which live within animal cells and are transmitted through cytoplasmic inheritance, exhibit distinct population structures with small population sizes and no recombination. These characteristics lead to the accumulation of mildly deleterious mutations. The study compares the evolutionary rates of 16S rDNA sequences from five independently derived endosymbiont clades to those of free-living relatives, finding significantly faster evolution in endosymbionts. For *Buchnera*, coding genes show even faster evolution and lower ratios of synonymous to nonsynonymous substitutions compared to free-living bacteria. This concentration of the rate increase at nonsynonymous sites supports the hypothesis that substitutions are deleterious in terms of polypeptide function. The observations are best explained by the combined effects of Muller's ratchet in small asexual populations and mutational bias. The loss of a repair gene and overproduction of a chaperonin in *Buchnera* may reflect compensatory evolution. An alternative hypothesis involving selection on genomic base composition is contradicted by the observed pattern of substitutions. The findings suggest that endosymbionts may undergo long-term deterioration due to mutation accumulation, potentially limiting the long-term fitness of their hosts.