Infestation with blow fly larvae (Protocalliphora (Trypocalliphora) braueri Hendel) was pathogenic to marsh wren (Cistothorus palustris) young. The mechanism of pathogenicity was Pseudomonas spp. infection of subdermal myiasis-induced lesions and subsequent sepsis. Neither internal organ involvement nor muscle destruction was seen on necropsy of fledglings. Multifocal hepatic necrosis was seen histologically and Pseudomonas sp. was isolated from myiasis sites, liver, and peritoneal cavities.