Ten bitter gourd genotypes were evaluated under field conditions for resistance to melon fruit fly, Bactrocera cucurbitae (Coquillett) during 2013-2014 rabi season at Department of Agricultural Entomology, College of Horticulture, Thrissur. VKB-196 with 9.13 per cent fruit damage was found to be the least susceptible to melon fly infestation, followed by CO-1 and IC-0596983, with 10.17 and 14.82 per cent fruit damage respectively. The varieties Priya, Priyanka, VKB-130 and Preethi with 49.08, 50.99, 53.14 and 64.56 per cent damage to fruits were identified as highly susceptible. Investigations into the morphological bases of resistance revealed that spine density and flesh thickness were negatively and significantly correlated with both per cent fruit damage (-0.81 and-0.69 respectively) and larval density (-0.74 and -0.69 respectively). Among the biochemical factors, peroxidase activity showed significant negative correlation with number of maggots per fruit (-0.72) as well as per cent fruit damage (-0.80), while protein & moisture content showed positive correlation.