Abstract.1.Callaspidia defonscolombeiwas found to be a common, solitary, thelytokous parasitoid of aphidophagous syrphid larvae. In 1977 it was bivoltine with adults being most numerous in early August and late September. It is probably polyphagous.2. An analysis of host searching behaviour suggests that females respond to an aphid odour bringing them to the region of an aphid colony.3. Syrphid hosts are located within the aphid colony by response to a contact chemical. There is a host size preference, only larger second and third instar larvae being attacked.4. Females inject a venom, which temporarily paralyses host larvae, and place the egg inside a cerebral ganglion.