The carrot psyllid, Bactericera trigonica, is a vector of 'Candidatus Liberibacter solanacearum' which infects carrots and many of solanaceous plants. The psyllid was first reported in Spain, then it was rapidly spreading throughout the Mediterranean countries, where it causes considerable damages to carrot crops. To minimize the spread of psyllid species as well as the associated fastidious bacteria, field inspections have been carried out in order to seek for a natural enemies of B. trigonica. Field surveys revealed the presence of a new eulophid parasitoid for the first time in Tunisia attacking larvae of B. trigonica, the main responsible for the significant losses in carrot fields. In order to study the parasitism rate of this parasitoid, a weekly sampling of carrot plants carrying the psyllids and its parasitoids was performed in carrot fields. The parasitism rate and the preferential stage for this parasitoid were also determined under laboratory conditions. The result shows Tamarixia pronomus, preferred late (4th and 5th) instar larvae to early (2nd and 3rd) instar larvae of B. trigonica. There, the parasitism average of late instars was estimated to average 80% and approached 100%. It should be noted that parasitism rates varied by season, with the spring showing the highest averages. Further efforts are warranted to enhance the biological control of B. trigonica and thereby reduce psyllid populations and the spread of 'Candidatus Liberibacter solanacearum'.