The biological relationships of lichenicolous fungi and their hosts are studied in four members of the Dacampiaceae. Infections of the lichenicolous species Dacampia engeliana induce in its host Solorina a thallus morphology similar to that found in the lichenized D. hookeri. D. engeliana, however, differs microscopically by hyphae growing inside those of Solorina in the algal layer and the medulla. Pyrenidium actinellum develops short haustorial structures into the hyphae of the host. These haustoria are characterised by finger-like outgrowths into the host cells. Clypeococcum hypocenomycis forms dense brown mycelia in the upper cortex of the host, which is progressively destroyed. These lichenicolous parasites have a preference for the fungal symbiont of the host lichen and, in contrast to several other lichenicolous fungi, the algal partner is not primarily affected. The various types of infection modes found in the family Dacampiaceae suggest that a great variation of infection modes may exist in further groups of lichenicolous fungi. Other infection types are included in the discussion.