SUMMARY: The effect of vegetative (heterokaryon) incompatibility on the transfer of a suppressive cytoplasmically determined condition, vegetative death, from carrier to normal strains of Aspergillus amstelodami has been investigated. Cytoplasmic transfer was reduced to 15% by vegetative incompatibility compared with 100% transfer in compatible combinations. Successful transfer in incompatible combinations involved donors and recipients whose incompatibility was determined by a single gene, and transfer was completely prevented between strains differing for more than one incompatibility gene. These results support the hypothesis that vegetative incompatibility serves as a cellular defence mechanism against genetic infection by stopping the spread of viruses and other suppressive cytoplasmic determinants from strain to strain in nature. Vegetative incompatibility is likely to be important in determining the specificity of virus-host interactions in fungi.