Perceptions regarding bush thickening in Namibia are rarely tested. It is often thought that bush thickening species such as Acacia mellifera subsp. detinens grow rapidly and recruit often. We estimated the growth rates and mortality patterns of 31 A. mellifera subsp. detinens shrubs in the arid Highland Savanna vegetation type in Namibia that were less than one metre tall in 1972, by remeasuring their heights in 2007. Some of these shrubs' heights were also remeasured in 1988. Growth rates were variable but on average individuals grew 3.19 cm in height per year. Growth was slower between 1972 and 1988 (2.67 cm per annum) than between 1988 and 2007 (3.85 cm per annum) for ten individuals remeasured in 1988. Based on these data, individuals of 1.5 m height are likely to be approximately half a century old, individuals of 2 m height 65 years old and individuals of 4 m height well over a century old. Mortality was high over the 35-year period (61.3%). Mortality between 1972 and 1988 was higher (45.2%) than between 1988 and 2007 (29.7%). These results support other studies that suggest that A. mellifera subsp. detinens is a very slow-growing species in the Highland Savanna, and that mature bush thickets observed today were mostly in existence as mature bush thickets half a century ago. Acacia mellifera subsp. detinens is a long-lived species, living for well over a century, but is susceptible to drought. Further implications of these findings are discussed. Keywords:Acacia mellifera subsp. detinens, bush encroachment, drought, growth rate, Highland Savanna, mortality, Namibia, rainfall, recruitment, saplings, seedlings, vegetation.