Mycosphaerella vespa sp nov. is described from diseased leaves of Euca
lyptus from south-eastern Australia. It is associated with small, circ
ular to irregular lesions that have red-brown margins and amphigenous
pseudothecia. These characters, in combination with ascospore morpholo
gy and germination, distinguish M. vespa from previously described spe
cies of Mycosphaerella on eucalypt leaves. Lesions associated with thi
s pathogen were often found to be harbouring wasps and this observatio
n is discussed.