Stalk synthesis in Caulobacter crescentus is a developmentally controlled and spatially restricted event that requires the synthesis of peptidoglycan at the stalk-cell body junction. We show that the beta-lactam antibiotic mecillinam prevents stalk synthesis by inhibiting stalk elongation. In addition, mecillinam causes an increase in the diameter of the stalk at the stalk-cell body junction. We describe two mutations that confer resistance to mecillinam and that prevent stalk elongation. These mutations are probably allelic, and they map to a locus previously not associated with stalk synthesis.