After i.v. injection of 25 mg/kh/BW and 50 mg/kg/BW to children age 5--6, no difference in the pharmakokinetic action can be found. The pattern of the plasma concentration allows the assumption of a 2 compartment model. There is a decrease of the plasma concentration in the steady state with a half life of 1,6 and 1,7 hours resp. The total volume of distribution is 30% of the body weight. Fosfomycin is eliminated by glomerular filtration only. The drug is not metabolished, as 98% and 95% of the dosage are recovered in active form in the urine. In premature and newborn babies there was a 1 compartment model assumed after i.v. administration of 25 mg/kg/BW. The distribution volumes with 41% of the body weight considerably greater then in older children. Elimination is again by glomerular filtration only. Plasma levels are decreasing considerably slower so that less of the drug is excreted in the 24-hour-urine. There is a sufficient penetration of the drug into the CSF. Bactericidal levels are reached within 48 hours by cumulation of the drug.