Two romaine lettuce cultivars, ‘Valmaine’ (resistant) and ‘Tall Guzmaine’ (susceptible) were evaluated to study survival and development of two lepidopterans: beet armyworm (BAW), Spodoptera exigua (Hübner) and cabbage looper (CL), Trichoplusia ni (Hübner). Larval mortality of BAW and CL was significantly higher on resistant ‘Valmaine’ than on susceptible ‘Tall Guzmaine’. Average weight per larva after 8 days on ‘Tall Guzmaine’ was eight times (BAW) and two times (CL) that of larvae on ‘Valmaine’. Significant reduction in larval growth on ‘Valmaine’ compared to that on ‘Tall Guzmaine’ resulted in 5.8 (BAW) and 2.6 days (CL) increase in larval duration. Average pupal weight of BAW and CL was 1.38 and 1.19 times less, respectively and pupae took longer time for eclosion on ‘Valmaine’ than on ‘Tall Guzmaine’. Resistant ‘Valmaine’ also caused significant reduction in pupation and adult emergence of BAW and CL, with surviving adults showing significantly lower weights and lower oviposition rates than those on susceptible ‘Tall Guzmaine’. Sex ratio was male biased on resistant ‘Valmaine’ (BAW, 1.20:1; CL, 1.18:1), and female biased on susceptible ‘Tall Guzmaine’ (BAW; 1:1.22, CL, 1:1.20). Feeding behavior of the two species differed on ‘Valmaine’ and may explain the superior performance of CL compared to BAW.