We studied the Armillaria population in a forest of natural pedunculate oak subject to periodic flooding and showing symptoms of oak decline. The study area was in Wołów Forest District in the Oder River Valley flood plain. Armillaria DNA was isolated by direct extraction from rhizomorphs and mycelium in soil and root samples. Armillaria species were identifi ed by PCR-RFLP of IGS-1 rDNA using the restriction enzymes AluI, TaqI, MspI, and BsmI. Rhizomorphs were found in 10 out of 12 sites. The number of soil rhizomorphs collected (0-28 in 100 kg of soil per sampling site) often differed significantly between sites. The mean number of soil rhizomorphs per sample was signifi cantly less in flooded sites (0.30 or 0.46) than in the non-flooded control (1.16), and insignifi cantly less in sites flooded once (0.30) than in sites flooded twice (0.46). Comparison of the RFLP patterns for Armillaria samples from the oak forest with those for Armillaria testers showed them to be A. borealis (two DNA samples), A. cepistipes (seven samples), A. gallica (150 samples), and A. solidipes (one sample). RFLP patterns for Taq1 were unfamiliar, since this enzyme has rarely been used, while A. cepistipes gave a rare pattern with AluI, known previously only from Japanese isolates of this fungus. Since AluI and TaqI each produced a similar pattern for A. borealis and A. solidipes, an additional functionally efficient enzyme, BsmI, was used. These results and those from other Armillaria population studies showed that flooding affected the abundance of soil rhizomorphs, but not the diversity of Armillaria species. The decreased abundance of Armillaria rhizomorphs in soil subjected to flooding may be indicative of the flooded soil syndrome in which general loss of fungi occurs. Flooding may still be considered, however, to be a short-term inciter and stress inducer, occurring between “primary” factors and “secondary” agents that predispose the tree to infection. This is the first report on populations of Armillaria in declining oaks in flood-plain forests in the northern hemisphere.