Mating system parameters and genetic diversity were examined for five populations of the endangered shrub Grevilleaiaspicula (Proteaceae). Controlled pollinations show that G. iaspicula has an effective self-incompatibility system and little potential for agamospermy. This is reflected in uniformly high multilocus outcrossing rates (tm=0.96–1.00). However, average paternal diversity within open-pollinated sibships is low (rp=0.31–0.54), suggesting that mating within populations is quite restricted. Despite the small size of most populations (four of the five populations studied have fewer than 20 reproductive individuals) the species still possesses moderate to high allelic richness (A=1.6–2.5). Interpopulation genetic differentiation is high (D=0.04–0.32), suggesting that gene flow is limited, even among populations separated by only a few kilometres.