Fall armyworm,Spodoptera frugiperda(J. E. Smith), is an early season production constraint of sorghum,Sorghum bicolor(L.) Moench, and maize,Zea maysL., in Central America. Because fall armyworm populations decline after a brief buildup on landrace sorghum, this study was conducted to determine whether antibiosis was a mechanism of resistance. Three fall armyworm feeding experiments were conducted in the laboratory using whorl-leaf material collected daily from sorghum. Experiments began with neonate larvae once plants had lived for 30–40 d. Mortality, pupal weight, generation time, intrinsic rate of increase (rm), and relative fitness were used to make inferences about antibiosis. Experiment 1 determined that the moderately resistant check, 1821 c.m. (rm= 0.31), was comparable to the susceptible check, ‘Huerin Inta’ (rm= 0.30), thus indicating that its mechanism of resistance was probably larval nonpreference. However, landrace San Bernardo III (rm= 0.27) significantly suppressed fall armyworm population density increase. Experiment 2 confirmed resistance in landrace San Bernardo III and showed that its resistance could be combined with that of inbred line TAM428 to produce an enhanced level of antibiosis. The mechanism of resistance in ‘AF28’ also was shown to be antibiosis. Experiment 3 indicated that antibiosis may be widespread in land race populations and that it was manifested differently than in ‘AF28’. Cohorts raised on three landrace accessions attained significantly lower pupal weights (184–191 mg/larva−1) than those raised on ‘AF28’ (216 mg/larva−1). They also suffered significantly higher mortality on ‘AF28’ (42%) than those raised on landrace populations (16–30%). Antibiosis was detected in the landrace accessions San Bernardo III, Hilate-179, Piña-61, and Lerdo-104. A hypothesis, based on differential selection and increased selection pressure brought about through intercropping with maize, is presented to explain the development of antibiosis in these landrace populations.