Median lethal doses (LD50's) were determined for geographical isolates of nuclear polyhedrosis virus (NPV) in fall armyworms (FAW),Spodoptera frugiperda,from several locations in the Western Hemisphere. LD50's ranged from 1.36 to 16.05 and from 0.85 to 11.93 polyhedra per insect in 1983 and 1984, respectively. LD50's for NPV from Louisiana were significantly lower than for other isolates in 1983, and in 1984 LD50's were higher in FAW from Brazil than from other locations. Two methods of cluster analysis of the results indicated that FAW populations in Louisiana were most similar in NPV susceptibility to FAW from Texas, and that populations from around the Gulf of Mexico and Puerto Rico were more similar to one another than to FAW from Brazil. In 1984 the geographical distance between the NPV isolate and FAW colony was significantly (P<0.01) and positively correlated with the LD50, but similar analyses were not significant (P<0.05) for 1983 or the combined data.