The efficiency at which tomato spotted wilt virus (TSWV) was transmitted by adult Frankliniella occidentalis that ingested the virus at different larval ages was determined by a petunia leaf-disk assay and enzyme-linked immunosorbent assay (ELISA). The results show that 0- to 2-day-old larvae (mostly first instars) can acquire TSWV, which suggests that after ingestion and accumulation of virus they were subsequently converted into transmitters in the adult stage. Older larvae (second instars) failed to acquire TSWV, because as adults they did not transmit TSWV. The relation between age at which virus acquisition took place and the amount of virus ingested was analyzed by ELISA. Ingestion of viral antigen increased with age of larvae, and the amount of virus ingested could not be correlated with the development of infectivity in the thrips. The finding that only first-stage larvae might be able to acquire TSWV is crucial for understanding the epidemiology of tomato spotted wilt disease.