Coat protein of raspberry ringspot nepovirus (RRV) migrated during electrophoresis in a discontinuous buffer system as two components which were capable of interchanging during this procedure. However, only one component was formed during electrophoresis in a continuous buffer system. Treatment of the protein before electrophoresis with iodoacetamide, which alkylates sulfhydryl groups, prevented the formation of the faster migrating electrophoretic component. Thus the faster migrating component is probably an artifact caused by S-S bond formation. Although the coat proteins of two other nepoviruses did not behave in this way, the results obtained with RRV emphasize the need for caution in interpreting electrophoretic heterogeneity in virus proteins. The estimated molecular weight of RRV coat protein was 57,000.