A total of 1439 Escherichia coli isolates from sick animals were received from the Spanish Network of Veterinary Antimicrobial Resistance Surveillance (VAV) from 1997 to 2001. Antimicrobial susceptibility tests were performed and diminished susceptibility to cefotaxime and ceftazidime was identified in 2.5% and 2.8% of the isolates, respectively. Beta-lactamase characterization was carried out in the group of 20 E. coli isolates with both characteristics. The MIC ranges of different beta-lactams showed by these 20 isolates were as follows (in microg/ml): ampicillin (64-->256), amoxicillin-clavulanic acid (4-64), ticarcillin (8-->128), cefazolin (32-->256), cefoxitin (4-->128), cefotaxime (1-64), ceftazidime (2-->64), ceftriaxone (0.5-64), imipenem (< or = 0.06-0.25), and aztreonam (2-->32). TEM, SHV, CMY, and FOX beta-lactamase genes were analyzed by PCR and sequencing. The beta-lactamase genes detected were the following ones (number of isolates): bla(TEM-1b) (3), bla(TEM-1a) (1), bla(TEM-30f) (2), bla(TEM-1b) + bla(CMY-2) (2), and bla(SHV-12) (1). Sequences of the promoter and/or attenuator region of the chromosomal ampC gene were studied in all the 20 isolates. Mutations at position -42 or -32 were detected in 16 isolates and these mutations were associated with the presence of a TEM type beta-lactamase in 6 isolates. Besides, a high variety of plasmidic beta-lactamases was detected including TEM-30 and CMY-2. To our knowledge, this is the first time that TEM-30 beta-lactamase has been detected in E. coli isolates of animal origin.