To examine the effects of cefoxitin on bacterial growth and cell morphology, two pairs of Bacteroides thetaiotaomicron strains (238, 238 m and 1186, 1186 m) with different susceptibilities to this antibiotic were investigated in the present study. B. thetaiotaomicron 238m and 1186m were resistant laboratory mutants originating from the susceptible wild-type strains B. thetaiotaomicron 238 and 1186, respectively. It has been shown, in a previous study, that the mutant strains had alterations in their penicillin-binding proteins (PBPs) as compared to the parent strains. In the present study, strains 238 and 238m presented almost identical genomic fingerprints by PCR, so did strains 1186 and 1186m, which indicates that the parent and mutant strains have similar genomic background. In comparison with the parent strains, the growth rate of mutant strains was slower in cultures without antibiotic. The growth patterns challenged with cefoxitin were also different between the parent and the mutant strains. In case of the morphological responses to cefoxitin, the mutant strains were more resistant to the effect of cefoxitin than the parent strains. In conclusion, the growth patterns and the morphological changes induced by cefoxitin, of the investigated strains, were associated with the properties of PBPs. The resistant mutants with deficiency in PBPs grew slower than the susceptible parent strains, and cefoxitin caused filamentation at sub-MIC in B. thetaiotaomicron.