After penetrating through the body wall into the haemocoel of Bradysia paupera, the fertilized female of Tipius sciarae increased in size and slowly expelled the enlarging uterine cells through the vulva.

Within 7 days of penetration, the females were mature and began laying eggs into the haemocoel of the host. The eggs hatched in 3 days and, within 2 weeks, the host–s body was swarming with juvenile nematodes. The juveniles moulted three times in the body cavity of the host and 4th-stage forms emerged through ruptures in the intestine or body wall (in larval hosts) or were deposited on the surface of the soil (by adult female flies). They then moulted to adult forms while remaining ensheathed in their last juvenile cuticle, mated, and the fertilized infective females were ready to enter a new host.

Most parasitized fly larvae died before reaching the pupal stage but some emerged as adults, still carrying the nematodes within them. All parasitized adult flies were sterile. Infested larvae had smaller fat bodies and adult histoblasts than normal larvae and took twice as long to develop.

Preliminary tests suggested that this nematode may be useful in controlling sciarid gnats in glasshouses.

T. sciarae (Bovien) and T. gibbosus (Leuckart) were compared.

This work was done at Rothamsted Experimental Station, Harpenden, Herts, England, while the author held a postdoctoral grant from the National Institutes of Health, Bethesda, Maryland. I thank Mr F. G. W. Jones for a place in the Nematology Department, Dr Audrey Shepherd for supplying the New Blue R stain, Dr J. B. Goodey for advice, and Dr K. Lindhardt, Denmark, for the loaning of the late Dr Bovien–s slides of T. sciarae.