The small brown planthopper (SBPH), Laodelphax striatelleus (Fallén), is a insect vector of Rice stripe virus (RSV) in temperate countries such as Korea, Japan, China, and Taiwan. As SBPH is able to overwinter successfully in these areas, RSV disease in subsequent rice fields has been believed to be endemic. In Korea, however, the RSV disease outbreaks have been observed mainly but not continuously at some western regions since 2001, caused a severe damage to the rice production. Although many efforts are underway to explain the outbreak phenomenon, the exact related factors are not known yet. In the meantime of the study on SBPH population dynamics in 2009, we catched unusually large numbers of SBPH adults by aerial net traps, maximally over 900, in early June at western coastal counties such as Taean, Seocheon, Buan, Sinan, and Jindo in Korea. Age distribution changes of SBPH in winter and post-winter seasons at some selected fields shows that the adults might be not related to overwintering population. The adults of overwintering population emerged from early April. Newly hatched nymphs of first generation were found from mid-May. In late May, just before the unusual catch of adults, the developmental stages of SBPH were mostly below 5th instars. This means that the big adult populations would be results of mass migration of SBPH abroad. We present also spacial distribution and host relationship of overwintering population as well as viruliferous rate changes of immigratory population.