In a herd of 120 two to eight months old calves kept at a Mexican Government experimental station in Ajuchitlan, Querétaro, weight loss and ptialism were observed. Upon a clinical examination, it was found that 31 (25.8%) of the animals disclosed papules in the oral and perioral regions. Biopsies from the affected tissues were studied with the light and electron microscopes. Serological studies and isolation of the virus were also carried out. A Pox virus was identified (240 x 100 x 150 +/- 7% nm) with the electron microscope. Dermatophilus sp. was occasionally observed. Bovine kidney monolayers, inoculated with affected bovine tissues demonstrated cytopathic effect up to the 4th serial passage. Inoculation with cell cultured infectious material in the oral submucosa (cell lysate) produced typical lesions of BPS on a heifer. Infectious tissues from this experimentally inoculated animal produced cytopathic effect in tissue cultured cells after 24 hours, and this last material was infectious for a second young heifer. Virus-neutralization tests, using an hyperimmune serum, disclosed a neutralization index of 1.5 logarithms. It was concluded that bovine papular stomatitis virus was the etiological agent.