The isolation and characterization of a mucoid, alginate-producing strain of Pseudomonas aeruginosa from a nonhuman host, namely, in chondroids from an equine guttural pouch, is reported for the first time. Pure cultures of P. aeruginosa 12534 were isolated from a 17-month-old pony mare with a history of chronic bilateral mucopurulent nasal discharge from the right guttural pouch. Transmission electron microscopy of chondroids showed mucoid P. aeruginosa growing as microcolonies within a matrix of extracellular material. On the basis of expression of the mucoid phenotype under different growth conditions, P. aeruginosa 12534 belongs to group 1 and resembles other isolates carrying the muc-23 mutation. The bulk of the extracellular material was characterized as being alginate by chemical and 1H nuclear magnetic resonance analyses, which showed that it had a composition similar to that produced by isolates of P. aeruginosa from human patients with cystic fibrosis.