The purpose of this study is to determine the etiology of abortions presented in a goat herd declared as free of brucellosis and vaccinated with RB51 located in Mexico. The serological diagnosis of brucellosis in 33 animals was performed. The study included three goats that aborted in the last third of gestation and 15 goats that gave birth normally; samples of milk and vaginal exudate were subjected to bacteriological study. All animals were negative for serological diagnosis, and isolation of Brucella melitensis was achieved in a single goat from vaginal exudate. However, the particularity is that this goat was negative to the card, indirect ELISA, and radial immunodiffusion tests. Isolation of a field strain was confirmed by biochemical test resistance to rifampicin and PCR. It is concluded that a goat which aborted in the last third of gestation was found spreading B. melitensis through vaginal discharge despite being vaccinated with RB51 and seronegative for brucellosis.