ABSTRACT The extracellular protein INF1 of Phytophthora infestans is a member of the elicitin family of protein elicitors known to induce a hypersensitive response on some solanaceous and cruciferous plants. The presence of INF1 elicitin in culture filtrates of 102 P. infestans isolates from 15 countries was examined. All tested isolates produced INF1 except five isolates collected in 1976 and 1977 from infected potatoes in East Germany (the former German Democratic Republic). Based on hybridization to the multi-locus DNA fingerprint probe RG57, all the INF1-nonproducing isolates were shown to belong to the clonal lineage US-1 that dominated world populations until the 1980s. Phylogenetic analysis of a set of European US-1 isolates using amplified fragment length polymorphism fingerprint data indicated that loss of INF1 production evolved independently in separate lineages within US-1. DNA and RNA blot hybridizations showed that INF1-nonproducing isolates still retain a copy of the inf1 gene, whereas little inf1 mRNA could be detected. Hypothetical interpretations of the evolution in a restricted geographic area of P. infestans lineages deficient in the production of a specific elicitor protein are discussed.