Nothing was known about the life cycle of Pseudacteon cultellatus Borgmeier. Because this species seems to be a promising candidate for biological control of fire ants, we studied several aspects of its development. We measured selected life history traits of the fire ant parasitoid Pseudacteon cultellatus as a function of (1) host species (Solenopsis invicta Buren versus Solenopsis richteri Forel), (2) temperature (22 versus 25°C), and (3) size distributions of available host ants (homogeneous small versus mixed sizes of workers). We found that larval, pupal, and total developmental periods of this phorid fly were between 12 and 18% longer on S. richteri than on S. invicta, and 11–19% longer at 22 than at 25°C. Although larval developmental times did not differ as a function of sizes of host offered, average time of development in the pupal stage was extended by 12% in the case of host size mixtures which included workers larger than the phorid’s preferred host size in comparison to homogeneously small ants. P. cultellatus exhibited a strong preference for small ants, especially when using S. invicta workers as hosts. We did not find a relationship between size of host and sex of emerging flies as previously documented for some other Pseudacteon species. P. culltelatus seems to be a promising species for controlling S. invicta as it develops faster in this smaller host, and because it prefers to attack small ants and does not need larger hosts to produce female flies. Thus, this P. cultellatus can attack the majority of ants in a colony without the risk of producing a male-biased sex ratio.