The composition of 16 natural whey cultures from 3 different Mozzarella chee se plants was investigated. They consisted mainly of lactic acid bacteria, coliform bacteria and yeasts. Micrococci, butyric and propionic acid bacteria only occurred occasionally. Lactobacillus lactis was the most common species of Lactobacillus while Streptococcus lactis and Str. thermophilus were the most common species of Streptococcus. Enteropathogenic Escherichia coli were always present. Different species of Leuconostoc and yeasts belonging to the genera Candida, Kluyveromyces, Debaryomyces and Brettanomyces were also isolated. Acidifying and proteolytic capacity of the strains showed that these activities were widely affected by tempe rature and type of milk (cow or waterbuffalo milk). Streptococci were the most active acid pro du cers at the cheese vat temperature (37 oC).