ABSTRACT: The municipality of Ji-Paraná, Rondônia, is one of the major dairy production areas in the north region of Brazil. Thus, it is important to evaluate infectious agents that have the potential to negatively affect productivity in the industry. The objective of this study was to investigate the prevalence of ' Candidatus Mycoplasma haemobos' by using a PCR-based detection method and correlate this with dairy herd variables (abortion frequency, weak calf birth rate, total cattle number, >24-month-old cow number, farm size, and production system) in family farms of the Ji-Paraná municipality, north region, Brazil. Blood samples were collected from 320 dairy cows located across 64 farms (i.e., five animals per farm) from September 2012 to November 2013. Overall prevalence of 'Ca. M. haemobos' was 64.2% and prevalence per herd was 95.3%; the number of >24-month-old cows in the farms studied correlated with ' Ca. M. haemobos' infection rates. Considering the importance of the dairy industry to the study area, additional investigations are necessary to evaluate the effect of chronic infection in these animals on milk production and herd health.