SummaryCasein breakdown and free fatty acids formation were studied in ten batches of La Serena cheese throughout a 60 d ripening period. A rapid degradation of αs1-casein occurred, with only 18·6% residual αs1-casein being detected in 60 d-old cheese, whereas 49·5% β-casein remained unattacked. High pH values, low NaCl-in-moisture concentrations and high moisture contents enhanced proteolysis. Lac-tobacilli and yeasts were the only microbial groups which significantly influenced αs1-casein breakdown, and only yeasts affected β-casein hydrolysis. No significant effect of chemical characteristics or microbial groups on FFA levels was detected, an average level of 12·7 mequiv./100 g fat being recorded for 60 d cheese. Rheological characteristics of the cheese reached their minimum values after 15–30 d of ripening and were dependent on αs1- and β-casein breakdown, pH value, NaCl-in-moisture concentration and moisture content.