Maggot therapy, also known as biosurgery, is an ancient method for the healing of chronic infected wounds. Although clinicians have reported on the beneficial activities of the Lucilia sericata larvae that have been used for healing chronic wounds, the selectivity of this therapy against the different pathogenic micro-organisms that are found in chronic wounds has never been analysed. In the present study, we have investigated the in vitro activities of larval excreta/secreta both against selected bacterial strains that frequently occur in chronically infected wounds, and against bacteria isolated directly from the larvae and their excreta/secreta. Additionally, the antibacterial activities were investigated in in vivo studies, by comparing bacterial diversity in wounds before and after the application of L. sericata larvae. In conclusion, larval therapy is highly recommended, particularly for the treatment of wounds infected with Gram-positive bacteria, like Staphylococcus aureus, but less so for wounds infected with Gram-negative bacteria, especially Proteus spp. and Pseudomonas spp. strains. Bacteria from the genus Vagococcus were resistant to the maggot excreta/secreta.