The natural mechanism of intercolony transmission of Thelohania solenopsae, a pathogen of red imported fire ants, Solenopsis invicta, is unknown. However, T. solenopsae can be transmitted by introducing infected brood into an uninfected colony. We hypothesized that the transfer of brood among colonies during intercolony competition may be a mechanism for the horizontal transmission of T. solenopsae. Male and female reproductive caste alates, collected during the initiation of mating flights from infected colonies, had T. solenopsae infection rates of 93 and 75%, respectively. In addition, 47 field-collected, newly mated queens that were reared in the laboratory established T. solenopsae-infected colonies that contained egg, larvae, pupae, and adults. Because T. solenopsae is transovarially transmitted, this indicated that infected founding queens generated infected colonies. A life span of ≤52 wk was documented for 81% of the infected queens and 59% for uninfected queens. To determine whether imported fire ant colonies can become infected with T. solenopsae via colony raiding, seven pairs of S. invicta colonies consisting of large, uninfected and small, infected colonies were given access to each other in the laboratory. T. solenopsae infection was detected in four of seven of the large colonies. In the four large, infected colonies, brood levels declined an average of 64% after 22 wk in contrast to a 116% increase in the controls. Thus, there was evidence that incipient, T. solenopsae-infected colonies could provide a source of inocula for the horizontal transmission of T. solenopsae through the transfer of brood during colony raiding.