Evidence from cytological behaviour, additive nucleotide bases in repetitive internal transcribed spacer regions of the rRNA-encoding DNA (rDNA), and amplified fragment length polymorphisms of total DNA showed that a new, aggressive Phytophthora pathogen of alder trees in Europe comprises a range of heteroploid-interspecific hybrids involving a Phytophthora cambivora-like species and an unknown taxon similar to P. fragariae. The hybrids' marked developmental instabilities, unusual morphological variability, and evidence for recombination in their internal transcribed spacer profiles indicated that they are of recent origin and that their evolution is continuing. The likelihood of such evolutionary events may be increasing as world trade in plants intensifies. However, routine diagnostic procedures currently in use are insufficiently sensitive to allow their detection.