Antibiotic dependence in clinical isolates has been reported, albeit rarely, such as vancomycin-dependent enterococcus and β-lactam-dependent Staphylococcus saprophyticus. We report herein a clinical isolate of β-lactam-dependent Bacillus cereus. A 16-yr-old female was admitted on 8 September 2005 with neutropenic fever during chemotherapy following surgical removal of peripheral neuroectodermal tumor. She had had an indwelling chemoport since August 2004 and experienced B. cereus bacteremia three times during the recent 3-month period prior to the admission; the bacteremias were treated with cefepime-based chemotherapy. On hospital days 1 and 3, B. cereus was isolated from blood drawn through the chemoport. The isolates were resistant to penicillin, ceftriaxone, and erythromycin, and susceptible to vancomycin and ciprofloxacin. The isolate of hospital day 3 grew only nearby the β-lactam disks including penicillin and ceftriaxone on disk diffusion testing. The β-lactam-dependent isolate required a minimum of 0.064 μg/mL of penicillin or 0.023 μg/mL of cefotaxime for growth, which was demonstrated by E test (AB Biodisk, Sweden). Light microscopy and transmission electron microscopy revealed a marked elongation of the dependent strain compared with the non-dependent strain. Prolonged therapy with β-lactams in the patient with an indwelling intravenous catheter seemed to be a risk factor for the emergence of β-lactam-dependence in B. cereus.