Evidence was obtained that copulation of Nippostrongylus brasiliensis did not occur until at least 104 h after rats had been subcutaneously infected with 1000 3rd-stage larvae. The majority of worms copulated for the first time approximately 120 h after infection.

Females usually, but not invariably, contained infertile eggs in the uterus before the first insemination occurred. Fertile eggs were not seen in the uterus until sperm had ascended from the vagina to the seminal receptacle, a process which appeared to take about 3 h.

On average each female in a normal mixed infection was capable of producing about 1000 fertile eggs every 24 h. Females recovered from rats on the 10th day after infection and transplanted into uninfected rats, were capable of producing an average of 3500 fertile eggs each without further insemination. When newly matured females, which had received a single full insemination, were isolated from males they produced on average, between 1000 and 1500 fertile eggs each.

From these observations it was concluded that in normal mixed infections females must copulate at least once every other day and probably once a day, in order to maintain fertile egg output. It was also deduced that females are capable of storing from 4000 to 4500 sperm in the seminal receptacle.

There was clear evidence of reduced reproductive efficiency in both male and female worms recovered from rats during the expulsion phase of the infection.

The author wishes to thank Professor J. J. C. Buckley and Dr J. A. McFadzean for their suggestions, criticism and encouragement and Miss G. Merchant and Mrs P. Counsel for able technical assistance.