The recombinant nucleopolyhedrovirus of Autographa californica Speyer (AcMNPV-AaIT) expressing the paralyzing toxin of scorpions (Androtoctonus australis Hector) kills caterpillars 30% faster than wild-type (wt) AcMNPV, and shows a great deal of promise as a bioinsecticide. Although it is generally believed that genetically modified organisms are less fit than their derived wild type and will not persist in the environment, a thorough assessment of the ecological risks associated with the release of AcMNPV-AaIT must be conducted before its commercialization. In this study, we focus on one aspect and compare the fitness of AcMNPV-AaIT and wt-AcMNPV during intra-host competition. Cabbage loopers (Trichoplusia ni Hübner) were synchronously or asynchronously infected with wt-AcMNPV and AcMNPV-AaIT, and the polymerase chain reaction was used to monitor the outcome of intra-host competition. There was no indication that AcMNPV-AaIT was less fit than wt-AcMNPV when T. ni were synchronously fed equal doses of each NPV. Serially passing the occlusion bodies had little effect on the persistence of AcMNPV-AaIT. After seven passages, the recombinant virus was found alone or with wt-AcMNPV in 71% (12/17) of replicates whereas wt-AcMNPV occurred alone or mixed in 88% (15/17) of replicates. Dose and synchrony of infection both affected the outcome of intra-host competition. The virus with the highest dose and the first one given to T. ni had a significant competitive advantage. Finally, the AaIT insert appeared to be stable because there was no evidence that it got deleted from the recombinant genome even after AcMNPV-AaIT was serially passed seven times in T. ni. As a complementary study, we also examined intra-host competition between two wild type NPVs, wt-AcMNPV and the single nucleocapsid (S) nucleopolyhedrovirus of T. ni (TnSNPV). Our results suggest that the two viruses are equally fit during intra-host competition. Serially passing the occlusion bodies did not influence the outcome but NPV dose and synchrony of infection were again strong determinants of intra-host competition.