Twenty-one isolates of Alcaligenes species were recovered from the respiratory tract of 16 patients at Texas Children's Hospital over a 1-year period. All but one were identified as Alcaligenes xylosoxidans; the remaining isolate was identified as Alcaligenes faecalis (formerly Alcaligenes odorans). Thirteen of 21 isolates were from the sputum of eight patients with cystic fibrosis (CF), two of whom were persistently colonized. The remaining isolates were recovered from intubated children. Patterns produced by repetitive-element-sequence polymerase chain reaction (rep-PCR), with use of either repetitive extragenic palindromic (REP) or enterobacterial repetitive intergenic consensus (ERIC) primers, showed that strains from different patients were distinct. This observation ruled out a common-source outbreak. Strains repeatedly cultured from the two persistently colonized patients over several months had identical rep-PCR patterns. We conclude that, similar to Pseudomonas aeruginosa, Alcaligenes species (most often A. xylosoxidans) colonize the respiratory tract of intubated children and of patients with CF. Colonization of patients with CF was associated with an exacerbation of pulmonary symptoms.