The objective of this paper is to evaluate the microbiological state and the dynamics of the mammary infections of organic farms in North Spain to discover if the high somatic cell count (SCC) observed in these farms is associated to a high incidence of mastitis. Microbiological cultures and SCC were performed in 8,496 foremilk samples collected from 160 cows in five representative organic farms from February 2006 to January 2008. Even though 79.3% of cultures were positive, only 21.2% of the total fit our diagnosis of mastitis (clinical, subclinical and chronic). The great prevalence of Corynebacterium bovis (teat canal-region pathogen) in the positive cultures that did not fit the mastitis diagnosis criteria (nearly 70%) compared with those that did (27%) was found to be related to lack of post-milking teat disinfection. The study prevalence of mastitis was 69.2% (66.7% subclinical mastitis, 27.8% clinical mastitis); the mean monthly prevalence was 47.4%; the mean monthly incidence was 12.9% and the mean duration of infection was 3.84 ± 3.98 months The high SCC in foremilk samples from old cows (three or more lactations) not diagnosed as mastitis compared to the heifers, reflects a worsening health status of the animals over time. When compared with the conventional sector in Northern Spain, these parameters indicate a poorer udder health in the studied organic herds with a high presence of chronic subclinical processes.