Changing the diet of five lactating cows and one nonlactating cow from high to low roughage induced milk fat depression in the lactating cows and altered the composition of the rumen microflora. While the numbers of lactic and propionic acid-producing bacteria increased, the numbers of Butyrivibrio spp. decreased. The numbers of lipolytic bacteria and the in vitro lipolytic activity of the rumen fluid were also decreased, as was the extent of hydrogenation of linoleic and linolenic acids combined in soybean oil incubated in vitro with rumen fluid. It is suggested that among the bacterial population in the rumen the vibrios, which were adversely affected by the low-roughage diets, may contribute significantly to both lipolysis and hydrogenation in the rumen.