The following fungi were tested for ability to grow on filter-paper cellulose as sole carbon source in pure culture: Ophiobolus graminis, Cercosporella herpotrichoides, Fusarium culmorum, Curvularia ramosa and Helminthosporium sativum. After 7 weeks incubation of the cultures at 22·5° C, the smallest loss in dry weight of the inoculated filter-paper was 0·8% for C. herpotrichoides, the highest 14·8% for H. sativum. The bearing of these results on factors determining longevity of saprophytic survival by the pathogens in infected wheat straw buried in soil is discussed. The low but significant degree of cellulolytic ability shown by C. herpotrichoides is considered to favour conservation of substrate reserves, and hence to permit this fungus to survive for a longer period (3 years or more) than can any of the other four pathogens listed above. The exceptionally high degree of cellulolytic ability manifested by H. sativum is thought to explain earlier observations that an abundant supply of soil nitrogen actually decreases longevity of this fungus in infected host residues, whereas it increases longevity of the other four pathogens.