The history and characteristics of two cells lines developed from primary explants of pupal tissue from the insect, Spodoptera frugiperda (J.E. Smith), are described. One cell line, IPLB-SF 21, was developed with hemolymph-supplemented medium and has been maintained continuously on the medium. The second cell line, IPLB-SF-1254, was developed with a medium containing a combination of vertebrate sera plus hemolymph and was adapted to hemolyphn-free medium at the 6th passage. The IPLB-SF-21 cell line has a population doubling time of 26 to 30 hr; the doubling time of the IPLB-SF-1254 line is 36 hr. The chromosomal morphology and distribution was typical of other lepidopteran cell lines. Serological studies showed that both cell lines have at least one antigen which also is common is tissue antigens from pupae of Spodoptera frugiperda.