Technical Abstract: Huanglongbing (HLB) is a devastating disease of citrus occurring in most citrus growing regions worldwide. It is caused by a phloem-limited, fastidious bacterium of the genus Candidatus Liberibacter, which is transmitted through insect vectors. HLB causes the decline of trees within few years after infection and is one of the greatest challenges the citrus industry is currently facing in Florida, where disease incidence has been reported to exceed 80 % in some areas. Typical symptoms of the disease are mottling and chlorosis of the leaves, similar to those caused by zinc and other nutritional deficiencies, as well lopsided and improperly colored fruit visible during the harvesting season. Despite much progress in the development of highly sensitive diagnostic methods, not much is known about the molecular and physiological processes resulting from the interaction of the pathogen with its citrus host. This study investigates the transcriptional responses of sweet orange (Citrus sinensis (L.) Osbeck ) to Ca. L. asiaticus infection under controlled conditions in the greenhouse using semi-quantitative RT-PCR analysis. Most notable was the large abundance of transcripts for glucose-6-phosphate/phosphate translocator and ADP-glucose pyrophosphorylase detected in leaves of infected plants. Both proteins are closely associated with starch biosynthesis in the plastids. Soluble sugar and starch content were measured in infected and non-infected leaves and the effect of liberibacter infection on carbohydrate metabolism in citrus is discussed.