Aspects of the biology of Proterodiplosis radicis were investigated. Field studies in the S.W. Ruahine Range were supplemented with laboratory experimentation. P.radicis is a root parasite of Metrosideros robusta A. Cunn. (MYRTACEAE,), and forms galls on the tree's fine rootlets. The host's phenology and the developmental stages of P.radicis are described. The insect passes through three distinct larval instars and pupates within the gall. Adult emergence continues from mid-February to March. The adults represent a non-feeding, short-lived (2-4 days) reproductive stage in the life cycle. Dispersal appears to be heterogenous and of little direct consequence to the population. Other aspects of the dynamics of the population were investigated and the findings collated in a life table. Greatest mortality occurs while the insect is without the protection of the gall, i.e. the adults, eggs and first larval instar. The first larval instar is of long duration and predation by eurytopic soil fauna probably effects the most significant regulation of the insect's numbers. The pupa is parasitised by a small ceraphronid while within the gall. P.radicis was found to be strongly host-specific. The subtleties of the intimate plant-insect association are discussed with reference to gall structure, metabolism and chemistry. The conclusion is drawn that P.radicis cannot be seriously implicated in the widespread debility of Metrosideros robusta.