Objectives: It has been suggested recently that intracellular bacteria surviving antibiotic treatment might serve as a reservoir for recurrent infection. The purpose of this study was to directly examine the location of Escherichia coli bacteria in the mouse bladder after treatment with mecillinam. Methods: The bladders were studied by use of colony counts, in situ hybridization and electron microscopy. Results: The bacterial counts in the bladder remained ∼103-4 cfu/bladder even after mecillinam treatment had finished, and re-growth in the urine was observed. In the bladder epithelium from treated mice, bacteria cells were occasionally seen, presumably representing intracellularly located bacteria. Conclusions: This is the first in vivo study indicating that during mecillinam treatment E. coli cells can penetrate the mouse bladder epithelium and persist.