Seeds were collected from each of twelve parent trees from each of six seed orchards of Acacia mangium in different locations in Vietnam to examine the relationship between growth and inbreeding in this species. Seedlings were raised and planted out in a field trial at Ba Vi in northern Vietnam. The inbreeding status (self-fertilized or outcrossed) of most of the seedlings was determined by DNA analysis. Heights were measured at 12 and 18 months, and diameter at breast height (dbh) at 18 months. There were significant differences in growth between the six orchard sources at 18 months. Progeny from two orchards based primarily on Papua New Guinea (PNG) provenances with high levels of outcrossing displayed the best growth, with mean heights of 3.1 m at 18 months, while an orchard also based primarily on PNG provenances but with predominantly selfed progeny, and another orchard based on Queensland provenances with 51% selfed progeny, displayed the poorest growth (mean heights of 2.0 m and 2.2 m respectively). Analysis of variance established that within the 32 families that included both selfed and outcrossed individuals, self-fertilized individuals were significantly (P < 0.001) slowergrowing than were outcrossed individuals, with selfs on average 15 % smaller in mean height and 16 % smaller in mean dbh at age 18 months, relative to outcrosses. The results demonstrate the need to minimise selfing in operational seed production for A. mangium plantations.