We studied the phenologies of phorid flies from the genus Pseudacteon; their host, the native tropical fire ant (TFA), Solenopsis geminata (F.); a second native ant, Monomorium minimum (Buckley); and the non-native red imported fire ant (RIFA), Solenopsis invicta Buren, at a site in Austin, TX. We took a census on an hourly basis from dawn until dusk once a month for a year (July 1998–June 1999). Previous studies have focused largely on the RIFA’s escape from natural enemies (e.g., phorid parasitoids) as an explanation for the ant’s competitive success. Here, we show that the RIFA has additional advantages over its native congener. We found the RIFA had a significantly greater association than the TFA with extremes in ambient temperature, relative humidity, and light levels. However, M. minimum was found at temperatures that were significantly higher than for the RIFA. Native TFA-specific phorids had activity patterns that were similar to their host’s. In addition, we documented the expansion of RIFA foraging territory in relationship to that of the TFA. This occurred throughout the year, so expansion was due to actual competitive takeover by mature RIFA colonies, rather than by establishment of new colonies. Predictions about the activities of South American phorids introduced to control the RIFA in North America are discussed.