The influence of an optical brightener, Tinopal LPW, on the activity of a purified genotype of the nucleopolyhedrovirus (SeMNPV) of the beet armyworm, Spodoptera exigua (Hübner), was determined in second to fifth instar (L₂-L₅) S. exigua. When mixed with viral occlusion bodies (OB) 1% Tinopal LPW significantly reduced the median lethal dose (LD₅₀) of the virus in all instars compared with insects treated with SeMNPV alone. Levels of enhancement, as determined by LD₅₀ values, ranged from 2.6- to 580-fold, depending on the instar. The greatest enhancement occurred on the two later instars, L₄ (70-fold) and L₅ (580-fold), which show a much higher resistance to SeMNPV infection than earlier instars. The median time to death (MTD) values were not significantly different in any instar among larvae treated with SeMNPV Tinopal LPW and those treated with SeMNPV alone. Larval development in SeMNPV Tinopal LPW treated larvae was retarded, in second and fourth instars, compared with controls or larvae treated with SeMNPV alone. The OB yields from SeMNPV treated larvae were almost 1.6-fold greater in second instars (9.3 × 10⁶ OBs/larvae), and 1.9-fold greater in fourth instars (1.9 ×10⁸ OBs/larvae), than those obtained in larvae treated with SeMNPV Tinopal LPW. The addition of 1% Tinopal LPW to the virus suspension did not alter the genotypic composition of viral progeny during four successive passages of the virus.