Regression equations for estimating fecundity of gypsy moth,Lymantria dispar(L.), from egg mass length were compared for a North American population and one from Sardinia, Italy. There was no significant difference in slopes and intercepts of the two data sets, indicating that the relationship between egg mass length and fecundity was the same for these two populations. A combined linear regression function was developed (log10number of eggs per mass = 1.48 log10, egg mass length + 0.44,r2= 0.71,P<0.0001). If subsequent comparisons of this regression with data from other populations reveal the same general relationship, measurement of egg mass length may be of general use as a simple, rapid, noninvasive field estimator of gypsy moth fecundity.