Aeromonas hydrophila (HG1)-specific RAPD-PCR fragments were investigated for their potential as DNA probes. From 20 RAPD-PCR fragment bands, it was found that two were specific to all isolates of Aeromonas hydrophila (HG1) tested. Cloning and nucleotide sequence determination of one of these bands showed that co-migration of similar sized amplicons had occurred and that this band (designated '7e') contained at least four fragments of different sequences. Three of these individual amplicons had a sequence specific to Aer. hydrophila (HG1) isolates. The sequence of one of these amplicons ('7e5') was used to design primers for a specific polymerase chain reaction (PCR). The specificity of the PCR was achieved using a modified hot-start procedure. The identity of the PCR amplicons was confirmed by high stringency hybridization with a digoxygenin-labelled 7e5 probe.