Resistance of herpes simplex virus to acyclovir is a problem of growing clinical importance. Acyclovir-resistance can be due either to mutations in the viral thymidine kinase gene or in the viral DNA polymerase gene. Although clinical resistance has most frequently been associated with thymidine kinase alterations, heterogeneity in clinical isolates has not been addressed frequently. The potential for such heterogeneity has been emphasized by a report describing a pathogenic clinical isolate containing within its population at least one thymidine kinase-proficient DNA polymerase mutant as well as mutants exhibiting thymidine kinase-deficiency (Sacks, et al., 1989). We provide here additional characterization of this isolate and speculations regarding its significance.