The social forms of the Red imported fire ants, which were initially discovered in Taoyuan County in 2003, were determined. Both polygynous and monogynyous colonies were found. The inclusion of both social forms suggested two possible scenarios of fire ant invasion. Fire ants queens or colonies of both social forms could have invaded Taiwan together at approximately the same time. Alternatively, fire ants of one social form could have invaded Taiwan before the other within a short time period. The scenario of lone invasion by either polygynous or monogynous colonies was ruled out due to male sterility of polygynous colonies and the presence of both social forms. The significance of the presence of both social forms of the red imported fire ants in Taiwan is also discussed.