Feeding tests with adult Exeristes comstockii (Cresson) showed that each of the 10 amino acids commonly recognized to be "essential" is required for egg production. Fecundity on a diet where only these 10 constituted the amino acid component equalled that on the control diet which contained 19 amino acids. Fecundity was reduced by 80% on diets in which Na+ was substituted for K+ and by one-third when K was the only mineral added. Omission of Mn2+ or Ca2+ from, or addition of Zn2+ to, the control salt mixture (Wesson’s) did not affect fecundity. Females fed the control (chemically defined) diet or its 10% dilution (sucrose content maintained) for 30 days in the absence of host larvae laid more eggs initially when hosts were supplied than did females fed 1% control diet, honeydew, or 10% sucrose for a similar period. Thus, for E. comstockii, only food with a relatively high quantity and quality of amino nitrogen fed during absence of host larvae will significantly increase initial fecundity when such hosts are later encountered. This knowledge affects treatment of parasites when being held before release in biological control programmes.