Red imported fire ants (RIFA), Solenopsis invicta is one of the most harmful invasive organisms on the earth, which have expanded from northern Argentina since 1930s. The economic impact of the ants in US estimates approximately US$5 billion annually and over 100 peoples killed. Since 2001 the ants had invaded in Australia, Taiwan, and China and potentially give magnificent damages for agriculture and native ecosystem. Fortunately, in Japan no RIFA introduction has occurred yet, however, we should establish several effective pest control techniques and monitoring systems before its introduction. In our research I focused on the cytogenetical and morphological aspects of RIFA in native (central and northern Argentina) and introduced areas (Taiwan and Florida) for understanding basic biological features to establish pest control. Here I provide following results,

(1) There were highly diversity of chromosome numbers and shapes.

(2) The diversity was mainly the differences of ploidy.

(3) As a result of FISH analyses and Ag-NOR staining, 18S rDNA were located different chromosomal locations and expressed each location.

(4) Morphologies of fire ants among populations were highly diversity.

I would like to discuss several factors of the highly cytogenetical and morphological diversities of the ants and exhibit the possibility of effects of hybridization and pesticides.