This book records the contributions presented at the XIIth Interna- tional Congress of Microbiology, Mycology Section, held in Munich on 3-8 September 1978. All the papers given at the Symposium (no. 33) on Fungal Viruses, and at the Round Table Discussion (RTD 1) are reported in full, and the paper on fungi as vectors of plant viruses by R.N. Campbell (who was unfortunately unable to attend the Congress) has also been included (Part A). Much of the current work with viruses in fungi involves genetic studies and virus-host gene interactions; for this reason, the Sympo- sium (no. 32) on Extrachromosomal Vectors in fungi has also been reported, in the form of abstracts from all those contributors who gave permission for this (part B). Authors' abstracts of posters relating to fungal viruses have simi- larlyl>een given (part C). Fungal viruses, or mycoviruses, can be defined as viruses that repli- cate in fungi, and since their discovery in 1962, considerable progress has been made towards an understanding of their biological and par- ticularly their physico-chemical properties. Present knowledge sug- gests that mycoviruses are usually latent in nature, and their biological manifestations can often be more readily studied as cytoplasmically inherited determinants than as viruses. An attempt has therefore been made in this book to bring together the most recent advances, not only in mycovirus research, but also in extrachromosomal determi- nants in fungi.