Development, longevity, and reproduction of fall armyworms,Spodoptera frugiperda(J. E. Smith), from Louisiana (LA) and Puerto Rico (PR) were studied on an artificial diet and on rice foliage. PR larvae developed more slowly than LA larvae and produced lighter adult females. This difference did not extend to adult longevity or reproductive parameters, but the next generation of first instars was smaller. Developmental parameters were not influenced by diet. The results suggest the possibility of a genetic difference between the two populations, indicating some degree of reproductive isolation.