Restriction endonuclease analyses of herpes simplex virus (HSV-1) DNAs revealed variability in the electrophoretic migration of specific Bam HI and Kpn I fragments. The variable regions of the genome map at the ends and near the ends of both L and S components, near the L-S junction region and in middle of the S component. The variability of the fragments was demonstrated by comparing the electrophoretic mobility patterns of DNAs from plaque-purified stocks originally derived from a common parental plaque-purified virus stock. Our analyses suggest that DNA sequences contained in the variable fragments can undergo accretion or deletion. So far the variability of the HSV-DNA molecules has been explained in terms of number of copies of terminally reiterated sequences (Wagner et al., 1978, Locker et al., 1979, Lonsdale et al., 1980, Davison et al., 1981). Our results may be explained by unequal crossing over, slippage, because of the presence of reiterated sequences, or by the presence of transposable elements.