Population dynamics of the hyperparasitic fungi Shaerellopsis filum on Puccinia coronata on Holcus lanatus were studied in field, greenhouse and laboratory conditions. Natural population studies in five plots (Reading and Shinfield) during 2005-2007 showed that S. filum increased after the rust in spring but did not show all the characteristics of an organism spreading by infection from one rust pustule to another. No significant differences were observed in the colonization pattern of S. filum between natural grass plots (Plots 2, 3 and 4) and established plots (Plots 5 and 6); colonization among the three layers of the leaves on K lanatus; or distribution of S. filum among the plots over the time period. However, rust was not present for the whole of 2006. When it reappeared in 2007, it was immediately colonised by S. filum, infection incidences reaching highest by June 2007.