The compost bedded pack dairy barn is an alternative loose housing system that has been implemented in the recent years. The objective of this study was to evaluate the effect of compost bedded pack on bacterial counts in bedding and milk in lactating dairy cows. A field study was conducted in Girona (Spain) between May 2013 and July 2014 in 2 commercial dairy farms (816 and 394 lactating cows). Cows were distributed in equal barns (90 ± 14 and 66 ± 7 cows) with different loose housing system: 2 compost-bedded-pack (compost) barns were compared with 2 conventional-bedded-pack (control) barns, 4 barns per farm. Compost barns were aerated twice daily, but less sawdust was added as bedding material. Farms were visited 6 times (spring, summer and autumn of 2013 and winter, spring and summer of 2014). During each visit, bedding material samples throughout each barn and milk samples of the total production of each barn of the morning and afternoon milking were collected. Data on milk composition (lactose, protein, fat and fat free total solids) and somatic cells count (SCC) were analyzed. Bacterial counts of bedding material were determined for mesophilic aerobic bacteria (total bacteria counts, TBC), other gram-negative, total coliforms, E. coli, Streptococcus spp., Strep. agalactiae, and Klebsiella spp. Bacterial counts of milk were determined for TBC, other gram-negative, lactose (-), total coliforms, E. coli, coagulase-negative staphylococci, Staph. aureus, Streptococcus spp., Strep. agalactiae, and Klebsiella spp. No significant effects in bacterial counts were found in bedding material (TBC 1.1 × 10¹⁰ ± 3.3 × 10⁹ cfu/g) or milk (TBC 5.1 × 10⁴ ± 9.3 × 10³ cfu/mL). Milk quality (SCC 240,000 ± 173,000 cells/mL) and composition did not change, except for lactose that was higher in compost cows than in control cows (4.91 vs 4.89 ± 0.008%, P < 0.05). These results suggest that udder health and milk quality were not compromised when housing lactating dairy cows were in compost barns. Saving of sawdust and manure storage costs without negative effect in milk quality, encourage the implementation of compost barns in dairy farms.