Streptococcus thermophilus INIA 463 became nisin-resistant after exposure in skim milk to subminimal inhibitory concentrations of nisin (1–3 IU/ml) for less than 2 h. Addition of 20 IU/ml caused a 4 log unit decrease in S. thermophilus counts of a culture not exposed previously to nisin, whereas no decrease was observed in the culture exposed to nisin for 2 h. Transfer of immunity genes as responsible for nisin resistance was discarded. The presence of extracellular or intracellular specific nisin-degrading enzymes was not detected in the nisin-resistant variant of S. thermophilus INIA 463. Nisin resistance was caused by the induction of a resistance mechanism. Transmission electron microscopy (TEM) revealed that the nisin-resistant variant of S. thermophilus INIA 463 had a thickened cell wall compared to the wild strain. Resistance to nisin was lost after one transfer (4 h growth) in nisin-free skim milk.