A method for rapid field estimation of gypsy moth,Lymantria dispar(L.), fecundity was developed and validated. A log-transformed linear regression (y= 1.58x + 0.29) was used to estimate number of eggs per mass from egg-mass length over a range of 8–54 mm in length and a range in fecundity of 26–1,509 eggs per mass (r2= 0.71). The model was tested against two independent data sets and data were pooled to produce parameter estimates that are independent of insect density, year, and locale.