Various studies have demonstrated significant inter-population variation in the ability of Cotesia plutellae to parasitize Plutella xylostella and also biochemical differentiation. In this context we have examined the morphology and sexual compatibility of 5 populations from different geographical origins with the aim of gaining better understanding of this diversity which will help in improving the use of Cotesia in biological control. The work was carried out under controlled conditions (25°C, 75% RH and 12:12 light dark regime). Important morphological differences between populations and between males and females of different populations were observed. Two population groups based on morphology and crossing are proposed. The populations which cross most easily have 2 morphological characters in common. It is suggested that the presence of symbiotic microorganisms is the origin of reproductive incompatibility. This work proposes a base line for studies on inter-population crossing and genetic variability in C. plutellae.