Gypchek is a gypsy nucleopolyhedrovirus (LdMNPV) product used for management of European gypsy moth (Lymantria dispar dispar L.) in the United States, primarily in areas where the use of broad-spectrum pesticides is not appropriate. Similar LdMNPV products are used in Russia for control of a flighted-female strain of Asian gypsy moth (Lymantria dispar asiatica Vnukovskij), an insect not yet established in the United States. Gypchek is a mixture of LdMNPV genotypic variants and is being developed further toward a single, high- potency genotype product that is effective against both European and Asian strains. We isolated 5 LdMNPV genotypic variants from Gypchek and, through diet incorporation bioassays, assessed their potencies for both a laboratory strain of European gypsy moth and a wild Asian gypsy moth strain. Bioassays conducted in the United States showed that 2 viral isolates, 122b and 122-HP, were about 3 times as potent as Gypchek against European gypsy moth. Bioassays conducted in Russia showed that 122-HP was as effective as a wild Siberian LdMNPV against a wild Asian (Siberian) strain of gypsy moth. Both 122-HP and 122b were shown to be at least as effective as Gypchek in killing European gypsy moth larvae when formulated at a high dose and sprayed on oak foliage in a ground-based field test. Overall results indicated that both 122b and 122-HP are potential candidates for further development as a single-genotype Gypchek product.