Technical Abstract: Chitinase modifying proteins (cmps) are proteases, secreted by fungal pathogens, which truncate the plant class IV chitinases ChitA and ChitB during maize ear rot. Cmp activity has been characterized for Bipolaris zeicola and Stenocarpella maydis, but the identities of the proteases are not known. Here we report that cmps are secreted by multiple species from the genus Fusarium, that Fv-cmp from Fusarium verticillioides is a fungalysin metalloprotease, and that it cleaves within a sequence that is conserved in class IV chitinases. Protein extracts from Fusarium cultures were found to truncate ChitA and ChitB in vitro. Based on this activity, Fv-cmp was purified from F. verticillioides. Amino-terminal sequencing of truncated ChitA and MALDI-TOF analysis of reaction products showed that Fv-cmp is an endoprotease that cleaves a peptide bond on the carboxyl side of the lectin domain. The amino-terminal sequence of purified Fv-cmp was determined and compared with a set of predicted proteins, resulting in its identification as a zinc metalloprotease of the fungalysin family. Recombinant Fv-cmp also truncated ChitA, confirming its identity, but had poor activity, indicating that recombinant protease does not mature efficiently from its propeptide-containing precursor. This is the first report of a fungalysin that targets a non-structural host protein and the first to implicate this class of virulence-related proteases in plant disease.