The genomic diversity among isolates of suid herpesvirus 1 (SHV-1) collected in the same herd and among clones from the same isolate was studied by restriction fragment pattern (RFP) analysis using BamHI. Tentatively defining a field strain as a transmissible entity, it was concluded that strains of SHV-1 commonly comprise distinguishable genomic variants. Contrary to the hypothesis of genomic lability, it is suggested that the pool of variants is sufficiently stable to specifically characterize a strain. The impact of the results on epizootiological methods based on identification of field isolates is discussed in connection with Aujeszky's disease in Denmark.