Technical Abstract: The Australian tree Melaleuca quinquenervia (Cav.) S.T. Blake is an invasive weed in wetland systems of Florida, USA. A biological control program targeting M. quinquenervia resulted in the simultaneous release of the gall-fly Fergusonina turneri Taylor and the nematode Fergusobia quinquenerviae Davies amd Giblin-Davis. Fergusonina Malloch (Diptera: Fergusoninidae) flies are gall formers that exploit plants in the Myrtaceae through a mutualistic association with nematodes in the genus Fergusobia (Currie) (Tylenchida: Neotylenchidae). With a limited number of founding individuals, a risk-spreading release strategy was employed in 2005 by liberating a total of 1996 adult flies across 7 locations in southern Florida. However, all release efforts failed to establish a viable population at any of the sites despite variation in location. In an effort to increase founding population size and improve phenological synchrony, 1432 individual flies and associated nematodes were released within a single M. quinquenervia stand during the early winter months of 2006-07. The population of F. turneri and F. quinquenerviae persisted at the field site for between two and three generations and, in accordance with the ca. 2-month generation time, emergence of F generation flies peaked in March, May and July 2007. Population growth rate increased with each succeeding generation up to the F3, after which the population went extinct. Both the F1 and F2 generations expanded spatially when compared to the distribution of their respective parental generations. The field population failed to spread after the F2 generation, with F3 generation galls found entirely within the spatial distribution of F2 galls. The release of F. turneri and F. quinquenerviae represent the first obligate mutualism used in weed biological control. Factors contributing to the failure of these species to establish are discussed.