The concentration of nitrogen in the foliage of Eucalyptus blakelyi strongly influenced both the total number of eggs laid and the rate of production of eggs by female Paropsis atomaria, a chrysomelid defoliator of Eucalyptus spp. Females feeding on foliage with low levels of nitrogen laid fewer eggs at a slower rate than those feeding on foliage with high levels of nitrogen. an increase from 1.5% to 4.0% of nitrogen in the foliage increased the total number of eggs laid by 500% and the rate of production of eggs by 400%. The rate at which females fed was not influenced by the concentration of nitrogen in their food. The viability of their eggs was negatively related with the concentration of nitrogen in the diet. Females feeding on foliage with low levels of nitrogen laid eggs which were significantly heavier than those of females on high nitrogen diets. Females on high nitrogen survived significantly longer than those on low nitrogen. In the field the concentration of nitrogen in the foliage of E. blakelyi occurs over a range shown to dramatically influence both the fecundity and rate of production of eggs of P. atomaria. Nitrogen may therefore play an important role in the population dynamics of this beetle.