Microplectron fuscipennis
, Zett., appears to be restricted for its host material to the European species of the Tenthredinid genus
Diprion
, Schrank. It has been recorded from
D. sertifer
, Geoffr.,
D. pini
, L.,
D. pallidus
, Kl., and, recently, from
D. polytomum
, Htg. The resting stage larva within the cocoon is attacked, oviposition occurring after the host has been paralysed. The present paper is an account of part of a more extensive programme of work on
Microplectron
which is being undertaken in view of the promising nature of the parasite and of its introduction into Canada. The biology of the species has been dealt with by Morris and Cameron (1935) and its reactions to changes in the physical environment by the present writer (Ullyett, 1936).