As parasitoid development is dependent on resources provided by a single host, the quality and quantity of resources its herbivore host obtains from different host plants can influence parasitoid fitness. Laboratory studies were conducted on several fitness parameters (developmental time, pupal weights, longevity, fecundity and sex ratio of progeny) of a larval parasitoid Cotesia vestalis (Haliday) (Hymenoptera: Braconidae) when its host Plutella xylostella (L.) (Lepidoptera: Plutellidae) developed on seven cabbage cultivars. In addition, population increase parameters of the parasitoid were compared on the various cultivars. The generation time of the parasitoid was shortest on Karabo (10.10 days) and Leano (10.38 days), and longest on Megaton (12.57 days) and Empowa (12.80 days). The heaviest C. vestalis pupae were recorded from Menzania (5.4 mg), Megaton (5.25 mg) and Beverly Hills (4.85 mg) and the lightest on Karabo (3.8 mg). Parasitoids reared on larvae that fed on Hollywood F1 lived the longest (2.28 days) followed by Menzania (1.94 days) and Beverly Hills (1.8 days), whereas those whose hosts fed on Leano lived the shortest period (0.83 days). Despite the parasitoids from Megaton hosts being bigger, their fecundity and number of female progeny per female (16.87 and 3.60, respectively) were lowest. Cotesia vestalis fecundity and daughters produced per female were highest on hosts fed on Menzania (38.00 and 9.13, respectively) and Beverly Hills (32.87 and 9.07, respectively). As a consequence, the net reproductive rate (R₀) and intrinsic rate of increase (r) were higher on Menzania (7.87 and 0.58, respectively) and Beverly Hills (8.29 and 0.62, respectively). Not only do these results suggest that the overall fitness of C. vestalis is higher on hosts developing on Menzania and Beverly Hills, the critical density of the parasitoid required to suppress the host population at a lower average density will be reached quicker on these two cultivars than on others.