Patients with cystic fibrosis (CF) may be colonized with unusual gram-negative bacilli whose identification is difficult and clinical impact unclear. We describe the clinical and microbiological features of five colonizations with organisms belonging to the recently described genus Inquilinus in CF patients. Isolates were identified from Burkholderia cepacia selective medium by means of 16S rRNA analysis. All of them were resistant to colistin, penicillins, cephalosporins, and monobactams but exhibited a remarkable susceptibility to imipenem. One of the five patients was transiently colonized with a nonmucoid isolate, whereas the four other patients were persistently colonized over the period of follow-up (8 to 21 months) with mucoid isolates. Pulsed-field gel electrophoresis of SpeI-digested genomic DNA was powerful for strain genotyping and demonstrated the clonality of Inquilinus sp. colonization for the two patients tested. Clinical evolution after the onset of Inquilinus was heterogeneous, but for at least one patient the lung function worsened and eradication of Inquilinus sp. was unsuccessful despite several imipenem courses. Finally, Inquilinus spp. may represent a new threat for CF patients due to their mucoid characteristic, their multiresistant pattern to antibiotics, and their ability to persist in the respiratory tract.