In 2000-2001, the frequency of metritis cases due to Klebsiella pneumoniae was investigated in Thoroughbred breeding mares in the Marmara region of Turkey, where most racehorses are reared. In addition, Thoroughbred stallions from the breeding centre in the Izmit district were examined for the presence of K. pneumoniae. Biotypes and capsule types of the isolates were detected, and their antibiotic susceptibilities were determined. Intrauterine swabs taken from 56 mares with metritis, and semen samples and swabs of urethral fossa from 27 stallions were investigated. K. pneumoniae was isolated from 13 (23.2%) mares and 2 (7.4%) stallions. Biotyping of the isolates was performed according to urease production, sorbose and dulcitol fermentation tests. Three of 13 K. pneumoniae isolates from mares were biotype-1, 5 were biotype-2 and 5 were biotype-3. Both isolates from the stallions belonged to biotype-3. The isolates were tested using antisera for capsule types 1 to 6 (Denka SEIKEN). Three (23%) of the 13 isolates obtained from mares were identified as capsule type-1 and all of them were biotype-1. The remaining 10, and the 2 isolates from the stallions were not identified as capsule types 1 to 6. With respect to antibiotic susceptibility tests of K. pneumoniae isolates from the mares and stallions, all isolates were sensitive to enrofloxacin, gentamicin and imipenem while being resistant to amoxicillin, erythromycin and rifaximin. Sensitivity to other antibiotics tested varied.