A man with acquired immunodeficiency syndrome (AIDS) developed zoster of the right arm which was resistant clinically to acyclovir. Varicella-zoster virus (VZV) was cultured from a skin biopsy performed at the beginning of acyclovir therapy (isolate 1) and after its failure (isolate 2). The emergence of acyclovir resistance during treatment was investigated by developing a simple and rapid drug sensitivity assay based on the plaque reduction reference method. This late-antigen synthesis reduction assay involved serial dilutions of cell-associated virus. The 50% inhibitory concentration (IC50) of acyclovir was 16 ± 7.5 µM for the susceptible reference strain OKA, in agreement with published data. The acyclovir IC50 increased from 6.5 µM for isolate 1 to 100 µM for isolate 2. In comparison with the sequence of isolate 1, isolate 2 had a single mutation consisting of a C to T change at position 907 of the thymidine kinase gene, which changed a glutamine codon into a stop codon at position 303 of the thymidine kinase protein. These results show the emergence of acyclovir resistance through a single previously undescribed mutation in the thymidine kinase gene, and confirm the heterogeneity of mutations inducing acyclovir resistance.