The tarnished plant bug, Lygus hesperus (Heteroptera: Miridae) is a pest on more than 150 plant species including field, fruit and vegetable crops. Our investigation on rose plants at Taif, Saudi Arabia observed that there are individuals of Lygus sp. during the flowering period. We morphologically identified these as Lygus hesperus. Consequently, this pest is considered as an exotic species in Saudi Arabia and it may be transformed from a minor to a major pest. We studied the length and sequencing of the ITS1 region and its flanking: 18S rRNA and 5.8S genes in this species. Biological and demographic parameters also were studied on two host plant species: green bean pods and rose flower buds. Sequencing of PCR product of ITS1 and its flanking in nine individuals of L. hesperus showed identical sequences for this fragment and obtained 743 bp for the complete ITS1. Biological data showed that the tested host source had an influence on all biological parameters except sex ratio, male longevity, preoviposition and postoviposition periods for females. The green bean pods as a host resulted in higher hatchability, shorter egg duration and nymphal development time, higher survival and fecundity, and longer female longevity. Therefore more favorable demographic parameters of L. hesperus were achieved on green bean pods where it had the shortest T and DT values as well as the highest GRR, r_m, λ and R₀ values. Additional biological data will be required on other plant hosts in the laboratory or field to estimate population growth and pest potential of L. hesperus in Saudi Arabia. Surveying of parasitoids and predators associated with this invasive insect pest are also desirable.