 There is established evidence that adult patients with Cystic Fibrosis CF may have altered antibiotic pharmacokinetics compared with non-CF patients. CeftolozaneTazobactam is a newly approved broad spectrum intravenous antibiotic, which has potent in vitro activity against multidrug resistant Pseudomonas aeruginosa, the most common pathogen implicated in CF pulmonary exacerbations. This study will determine the pharmacokinetics and tolerability of ceftolozanetazobactam in 20 adult CF patients admitted for a pulmonary exacerbation at one of 4 participating hospitals in the US. Patients will remain on standard of care IV antibiotics and receive 4-6 doses of ceftolozanetazobactam 3 grams every 8 hours. Blood will be sampled after the final dose to determine concentrations and pharmacokinetics of ceftolozane and tazobactam. Safety and tolerability will be assessed throughout the 3 day study. 