Abstract: The main microbial causes of IMI in heifers in a dairy farm with long history of high incidence of mastitis were studied in 53 pregnant heifers. Mammary secretions (52 samples) and milk (53 samples) were taken on day 5�5 before and day 10�5 after calving, respectively. Each sterile composite sample from 4 quarters was obtained, refrigerated and transported into the lab for culture and SCC. Results showed that in the pre calving samples, the most frequent bacteria found were: CNS, Staph. aureus, E.coli, Strep. dysgalactia and Strep. uberis at the rate of 71, 68, 57, 35 and 32% of the samples, respectively. In post calving samples, prevalence of CNS, Staph. aureus, E.coli, Strep. dysgalactia, agalactia, bovis and uberis was 69, 47, 18, 37, 24 and 5%, respectively. Streptococci were found in all the pre and/or post parturition samples. Yeasts were isolated from 7.7% of the pre and 9.43% of the post calving samples. The most prevalent CNS was Staph. chromogenes which was found in 20% of the pre and 28% of the post parturition samples. SCC in all the samples were higher than the local standards (2�10⁵), showing high contamination of the mammary glands with environmental and/or contagious microorganisms.