The suitability of random amplified polymorphic DNA for identification of Pythium aphanidermatum was investigated. Three oligoprimers were selected after testing three isolates of P. aphanidermatum and one isolate of P. ultimum with a total of 40 primers. The selected 10-mer primers were used with 20 isolates of P. aphanidermatum, four isolates of P. deliense, two isolates of P. ultimum, two isolates of P. irregulare and one isolate of P. paroecandrum. Most of the P. aphanidermatum isolates (13 of 20), were obtained from samples of Cucumis sativus, or water from cucumber greenhouses. The three selected primers gave identical fingerprints for 18 of the 20 P. aphanidermatum isolates, including all the isolates from cucumbers. Two of the primers gave fingerprints that could be used to differentiate between isolates of the Pythium species studied. The banding pattern of P. aphanidermatum given by the third primer could not easily be distinguished from the fingerprint of P. deliense. However, when used in conjunction with the other two primers, the third primer can be used to verify the identity of P. aphanidermatum.