ABSTRACT. Costa K.O., Alzamora Filho F., Costa J.N., Amorim C.R.N.,Yano T. & Conceição R.A. [Virulence factors of Escherichia coli isolated from calves with diarrhea in the region of Feira de Santana, Bahia.] Fatores de virulência das amostras de Escherichia coli isoladas de bezerros com diarreia na região de Feira de Santana, Bahia. Revista Brasileira de Medicina Veterinária, 36(4):430-436, 2014. Departamento de Ciências Biológicas, Universidade Estadual de Feira de Santana, BR 116 norte, Km 3, Feira de Santana, BA 44032-560, Brasil. E-mail: email@example.com Escherichia coli is a Gram-negative bacterium most commonly isolated from intra and extraintestinal infections in both man and in other animals such as pigs and calves. Six distinct groups of E. coli can be identified by the type of toxin and the clinical signals they produce. Among these, E. coli enterotoxigenic (ETEC) is the most commonly found and produces two types of toxin: heat-labile (LT-I and LT-II) and heat-stable (STa and STb). Another virulence factor of E. coli is the presence of adhesive antigens that enable the bacterium linking to host cells. Adhesins more frequently found in bacteria that produces diarrhea in cattle are F5 (K99), F17 and F41. The objective of this study was to identify by PCR virulence factors present in Escherichia coli strains isolated from diarrhoeagenic stool of calves from the region of Feira de Santana, Bahia, Brazil. Of the thirty isolated strains, twenty-eight (93.3%) were positive for at least one of the virulence factors studied in this work. Entorotoxin genes Stx-1 and LT-2 were present in 40% and 46.6% of the strains, while fimbrial adhesins genes F5 and F17, were present in 63% and 30% of the strains, respectively. Cytotoxic Necrotizing Factors (CNF1 and CNF2), STa and F41 genes were not found in any of the strains. Biological phenotypic characterization using in vitro cultivated Vero cells which indicates either the presence of Stx and LT genes were positive in 40% and 46.6% of the strains, respectively.