Pseudomonas aeruginosa is a frequent and virulent pulmonary pathogen in patients with cystic fibrosis. If colonization is not prevented, P aeruginosa becomes permanently established and nearly always mutates into a mucoid strain. The alginate-containing matrix of the mucoid strain is thought to allow the formation of protected microcolonies and provide increased resistance to opsonization, phagocytosis, and destruction by antibiotics. As a result, conversion to the mucoid phenotype is associated with a significant increase in morbidity and mortality. In the microbiology laboratory, mucoid P aeruginosa has a distinct Gram stain and culture appearance that can expedite its identification and facilitate appropriate patient management. Important aspects of the mucoid phenotype are reviewed.