Epidemiologic investigations were conducted during one year to evaluate the seroprevalence of the brucellosis at the ruminants in the region of Mostaganem. In the cattle, 3106 sampling of blood provided from 479 dairy farms was examined by means of the Rose Bengale test (RBT) and complement fixation test (CFT).The seroprevalence of global herd and individual were obtained respectively 3.34% (95% Confidence Interval [CI]: 1.73-4.95) and 0.97% (95% CI: 0.62-1.31). The rates of infection were significantly elevated in males compared to females.However, there is no significant relationship between the rates of brucellosis infection (p=0.66) and the different zones studied. The second investigation using RBT was conducted in the rearing area of small ruminants and involved 450 sheep and 287 goats. A significantly different overall individual seroprevalence was observed between ovine races (2.22%; CI 95%: 0.86-3.58) and goats (5.23%; CI 95%: 2.65-7.80). On the other hand, at the herd level, no significant difference of overall seroprevalence was observed between these species. The order of the RBT rates are respectively 7.69% (95% CI: 1.21-14.2) for ovine races and 17.5% (95% CI: 5.72-29.3) for goats.These results showed clearly the high rates of infection at the ruminants.