We studied the developmental performance of the large biotype of Pseudacteon obtusus Borgmeier, a promising biological control agent of imported fire ants. We measured selected life history traits of this parasitoid as a function of (1) host species (Solenopsis invicta Buren versus Solenopsis richteri Forel), (2) temperature (22 versus 28°C), and (3) size distributions of available host ants (big versus mixed-size classes of workers). Survivorship on S. invicta was much greater than that on S. richteri under most conditions. Developmental time was strongly influenced by temperature with total developmental periods shortened by 21–34% at the 6°C higher temperature treatment. However, developmental periods were weakly influenced by hosts across temperatures. We found that larval, pupal, and total developmental periods of this phorid fly were up to 6.2% longer on S. richteri than on S. invicta, although these periods depended on temperature. Total developmental time was slightly shorter (by 4.8%) on S. invicta than S. richteri at 22°C but longer (by 5.7%) on S. invicta at 28°C. The relationship between host size and sex of emerging flies contrasts with that of previously documented Pseudacteon species, in that males were produced from all host size classes while females only came from larger ants. Sex ratios favor females when a mixture of ant size classes were offered in comparison to when only big ants could be chosen. Pseudacteon obtusus seems to be a promising candidate for biological control releases in that in addition to its high host specificity, it develops better on red fire ants, attacks ants on trails and mounds, and chooses a worker size range that complements the two other Pseudacteon species already released.