Biodynamic (BD) agriculture is an organic farming system that relies heavily on compost as a fertilizer. Six herbal preparations are added to composting materials in order to make BD compost. Proponents claim these additions produce higher quality compost under farm conditions. In this study, BD compost preparations were applied to 3.5 t compost piles made of dairy manure and woodshaving bedding. Application of the BD preparations also requires 6 1 soil and 8 1 water; therefore control piles received the same additions of soil and water as BD compost piles, but no BD preparations. Biodynamic-treated composts maintained an average 3.4°C higher temperature throughout the eight-week active composting period, suggesting more thermophilic microbial activity and/or faster development of compost with BD treatment. Final samples were taken when active composting slowed and the piles entered a ripening stage. At the final sampling, BD-treated piles respired C02 at a 10% lower rate and had a larger ratio of dehydrogenase enzyme activity to C02 production. Microbial communities in the finished BD and control piles were differentiated by principal component analysis of microbial phospholipid fatty acids. Final samples of BD-treated composts also had 65% more nitrate than control piles. Biodynamic preparations thus effected discernible changes in compost chemical and microbial parameters.