Flies of the family Acroceridae are specialized internal parasitoids of spiders. We infected hatchlings of wolf spiders Pardosa prativaga (L. Koch 1870) (Araneae: Lycosidae) with larvae of Acrocera orbiculus (Fabricius 1787); most hosts were infected by a single larva, but others endured multiple infections of up to eight larvae. The infected spiders and a group of uninfected control spiders were raised in the laboratory for up to 23 weeks. We found that most (81%) spiders infected by only one larva were able to suppress the infection, whereas most multiple infections (73%) were “successful” (i.e., a larva emerged or was recovered by dissection, perhaps from a prematurely dead spider). Infected spiders had their survival reduced in proportion to the infection load, but the reduction was not significant if the infection was suppressed. Infected spiders had higher growth rates than uninfected, and growth stimulation was proportional to the number of initially infecting larvae and independent of whether the larva was suppressed or not. Due to these patterns, we suggest that growth enhancement results from the spider's mobilization of extra resources for combating the infection rather than parasitoid manipulation of spider growth. Spiders with multiple infections took longer to mature than uninfected spiders, and the pattern of instar durations was changed compared with that of control and singly infected spiders. As multiple infections were important for the parasitoid's success, we suggest that the parasitoid fly's habit of laying eggs in large clumps may be an adaptation to increase the chance of success via multiple infections.